Posters

ID 1
The atomic method for the Hubbard dimer
Renan Lira Nunes, Marcos Sergio Figueira da Silva
Universidade Federal Fluminense
The single-band Hubbard hamiltonian [1] is the simplest model of interacting particles in a lattice. It was developed by John Hubbard, who introduced the model to take into account the local electronic correlations in narrow energy bands. The model has a kinetic term that allows hopping of electrons between atomic sites and a term that considers the on-site interaction between electrons. This model is largely used in solid state physics to study magnetic properties of solids, insulator-metal transition (Mott transition) and high temperature superconductors. However, this is a problem that has exact analytical solution in a very few limiting cases and must be treated approximately or numerically. In this work, we propose a new methodology to analytically solve the Hubbard hamiltonian, mapping it into a two-site model (the Hubbard dimer), where one of the sites represents any of the atomic sites in the lattice and the other represents all of its first neighbours. To obtain the Green's function for the lattice, we employed the cumulant expansion technique, using as a 'seed' the exact two-site Green's function (the atomic method) [2]. From the Green's function, we obtained the density of states (DOS) and the occupation numbers as functions of the external parameters of the model, and compared the results with the Hubbard I approximation [1]. We also obtained a peak at the DOS, located at the chemical potential, which may be interpreted as a signature of the Kondo effect in the model, since it appears when the fundamental state is a singlet state of the dimer. The Kondo peak, in the Hubbard model, can only be obtained using more sophisticated approaches like the dynamical mean-field theory (DMFT) [3]. [1] J. Hubbard. Proc. R. Soc. Lond. A 1963 256. doi: 10.1098/rspa.1963.0204. [2] T. Lobo, M. S. Figueira and M. E. Foglio, Nanotechnology. 21, 274007 (2010). [3] A. Georges, G.Kotliar, W. Krauth, and M. J. Rozenberg. Rev. Mod. Phys. 68, 13. doi: 10.1103/RevModPhys.68.13.
 
ID 2
Majorana Fermion and bound states in the continuum on a cross-shaped quantum dot hybrid structure
D. Zambrano, J.P. Ramos-Andrade, P.A. Orellana
Universidad Técnica Federico Santa María
We show how transmission and density of states (DOS) behave when two superconductor nanowires are placed next to the ends of a quantum-dot (QD) chain, where the central QD is attached to normal conductors leads. Results in a single QD coupled to two Kitaev chains within the topological phase [1] and a T-shaped QD hybrid structure [2] suggest these kinds of system are strong candidates for qubits. We show how bound states in the continuum (BICs) arise as zero energy modes on conductance and DOS for different sets of system parameters showing evidence of Majorana fermions, and we also study how they behave for different numbers (even/odd) of QD in the cross-shaped structure. The authors acknowledge financial support from CONICYT, under Grant PAI-79140064 and 21141034. And from FONDECYT, under Grant 1140571. [1] L. S. Ricco, Y. Marques, F. A. Dessotti, R. S. Machado, M. de Souza, and A. C. Seridonio, Phys. Rev. B 93, 165116 (2016). [2] Wei-Jiang Gong, Shu-Feng Zhang, Zhi-Chao Li, Guangyu Yi, and Yi-Song Zheng, Phys. Rev. B 89, 245413 (2014).
 
ID 3
Detecting Majorana bound states coupling with an Aharonov-Bohm interferometer
J. P. Ramos-Andrade, P. A. Orellana, S. E. Ulloa
Departamento de Física, Universidad Técnica Federico Santa María, Chile, Department of Physics and Astronomy, and Nanoscale and Quantum Phenomena Institute, Ohio University, USA
In this work we consider a quantum dot (QD) connected to current leads arranged to mediate the interaction between two topological nanowires, both hosting Majorana bound states (MBS) at their ends. In an interesting system geometry, one nanowire has both ends coupled with the QD, forming an Aharonov-Bohm (AB) interferometer, while the other is placed nearby such that two MBS belonging to different nanowires can interact. We model the system using an effective low energy Hamiltonian, considering that the QD is embedded between metallic leads. Using a Green’s function formalism via the equation of motion procedure, we find that the conductance across the leads can show MBS signatures, i.e. half-maximum conductance at zero-energy, when both topological nanowires are connected, independent of the AB flux phase. We propose our findings to be used as a detector of the effective connections between independent MBS by monitoring the conductance while tuning the AB phase.
 
ID 4
Comparison between $\mathbf{k.p}$ Hamiltonians: range validity and accuracy of parameters.
Fabio Danielli Bonani, Carlos Maciel de Oliveira Bastos, Guilherme Matos Sipahi
São Carlos Institute of Physics-USP
The first-principles method based on density functional theory (DFT) plays a crucial role in the study and designing of new materials. However, the computational cost is a hard limit to the study of large-scale systems such as nanowires and mesoscopic systems. An alternative approach is the $\mathbf{k.p}$ method, that uses effective Hamiltonians determined through the use of the crystal symmetry. In this method, the matrix elements are parameterized and obtained either from experimental results or first-principles calculations. Despite of having been widely employed in the last 50 years, systematic studies addressing the determination of parameters or even the range of validity of these Hamiltonians are not common. In this work, we performed a systematic analysis of both, obtaining the parameters from a fitting of the secular equation of the Hamiltonian to the DFT band structure[2]. The band structure has been evaluated using hybrid functional HSE and the parameters were adjusted to obtain the experimental band gaps and spin-orbit splittings [1].  Using our framework, we analyse the electronic properties of GaAs and GaSb compounds, showing that for a region of same range around the $\Gamma$ point, GaAs presents higher accuracy than GaSb. The smaller energy separation of the remote conduction band of GaSb is responsible for this phenomenon.  Furthermore, comparing the $6\times 6$ and $8\times 8$ Hamiltonians with well adjusted parameters highlights the role of th interband parameter between conduction and valence bands (P), providing an improvement of the understanding of the results obtained with the $\mathbf{k\cdot p}$ method. [1] C.M.O. Bastos, F.P. Sabino, G.M. Sipahi, J.L.F. Da Silva. III-V Semiconductors from Hybrid-Density Functional Theory, submitted to PRB. [2] C.M.O. Bastos et. al., Semicond. Sci. Technol. 31, 105002 (2016)
 
ID 5
Defect-enhanced Rashba spin-polarized currents in carbon nanotubes
Andrea Latge, Hernan Santos, J. Alvarellos, L. Chico
Instituto de Física Universidade Federal Fluminense, Departamento de Física Fundamental - Universidad Nacional de Educación a Distancia - Madrid, Instituto de Ciencia de Materiales de Madrid - Consejo Superior de Investigaciones Cientificas.
Spin-dependent polarization of pristine carbon nanotubes with Rashba spin-orbit interaction has  been shown to be very sensitive to the symmetry of the tubes and the geometry of the setup. Here we present a detailed study of the role of defects on the spin-polarized currents in metallic carbon nanotubes due to an external electric field. We show that localized defects, such as adsorbed hydrogen atoms or pentagon-heptagon pairs, increase the Rashba spin-polarized current. Moreover, this enhancement takes place for energies closer to the Fermi energy as compared to the response of pristine tubes. Such increment can be even larger when several equally spaced defects are introduced in the system. The spin-dependent transport responses are quite sensitive to the geometrical position and orientation of the defects. We have explored several arrangements of defects, showing that for certain geometries there are flips of the spin-polarized current and even transport suppression. Our results indicate that spin valve devices at the nanoscale may be achieved via defect engineering in carbon nanotubes.
 
ID 6
Impurity effects in graphene/h-BN zigzag nanoribbbons
Andrea Latge, Carlos Leon, Márcio Costa
Instituto de Física Universidade Federal Fluminense
We analyze the electronic properties of a hybrid graphene-BN nanoribbon system, using a Hubbard model Hamiltonian within a mean field approximation. Due to different electronegativities of the Boron and Nitrogen atoms, an electric field is induced across the zigzag graphene strip, breaking the spin degeneracy of the electronic band structure. Optimal tight-binding parameters are found from DFT calculations. Edge potentials are used as corrections for on-site energies. We also investigate the changes of energy potentials at the BN-graphene nanoribbon interfaces. We analyze the effects of impurities along the graphene nanoribbon and through the interfaces and found that energy gap sizes may be properly engineered by controling the spatial doping process. Binding energy impurity calculations are used to study impurity diffusion processes along the hybrid nanoribbon system. We show that substitutional impurities may enhance half-metallic responses.
 
ID 7
Measurement of the effect of strain on the photoluminescence of WS2 monolayer blisters
Anna Carolina Marx Gonçalves, José David Hernandez Rivera, Ingrid Barcelos, Pierre-Louis de Assis, Paulo Sérgio S. Guimarães
Department of Physics, Federal University of Minas Gerais, CNPEM - LNLS, ’Gleb Wataghin' Institute of Physics, University of Campinas, Department of Physics.
Transition metal dichalcogenides (TMDC) are a class of 2D semiconductor materials that have been the subject of intense study in the past decade, for their potential use in a wide variety of ultra-thin electronic and optical applications. A particularly important feature of TMDC is their large band gap and exciton binding energies, leading to intense photoluminescence (PL) at room temperature. This PL lies in the visible range for XS$_2$ TMDC and in the infrared for XSe$_2$, with X being Mo or W, and is known to be strongly affected by factors such as the substrate on top of which the monolayer lies or mechanical strain applied to it. The band gap shift of a 2D semiconductor due to strain is a key ingredient for the fabrication of hybrid optomechanical systems with extremely low mass, that have the potential to exhibit very high zero-point coupling rate needed for the realization of optomechanical devices operating in a quantum regime. In this work we study the effect of strain on the PL energy of WS$_2$ monolayers suspended over a circular hole made in SiO$_2$, confining a small volume of air due to strong adhesion to the substrate. When we put the sample in a vacuum, low or high pressure environment, the pressure difference between both sides of the membrane generates a blister that strains the monolayer by applying a total pressure that is uniformly distributed. We performed room temperature micro-PL maps of monolayers suspended over circular holes of different diameters, in order to measured the effect of different amounts of strain for each given pressure difference. We also tested different hole shapes, to investigate how different clamping perimeters may generate strain hotspots that are useful to increase the hybrid optomechanical coupling. The authors acknowledge the financial support of FAPEMIG, CAPES and CNPq. The band gap shift of a 2D semiconductor due to strain is a key ingredient for the fabrication of hybrid optomechanical systems with extremely low mass, that have the potential to exhibit very high zero-point coupling rate needed for the realization of optomechanical devices operating in a quantum regime. In this work we study the effect of strain on the PL energy of WS22 monolayers suspended over a circular hole made in SiO22, confining a small volume of air due to strong adhesion to the substrate. When we put the sample in a vacuum, low or high pressure environment, the pressure difference between both sides of the membrane generates a blister that strains the monolayer by applying a total pressure that is uniformly distributed. We performed room temperature micro-PL maps of monolayers suspended over circular holes of different diameters, in order to measured the effect of different amounts of strain for each given pressure difference. We also tested different hole shapes, to investigate how different clamping perimeters may generate strain hotspots that are useful to increase the hybrid optomechanical coupling. The authors acknowledge the financial support of FAPEMIG, CAPES and CNPq.
 
ID 8
Optical characterization of buried MoS2 nano-islands fabricated with local anodic oxidation
Thales F. D. Fernandes, Pierre-Louis de Assis, Paulo Sérgio S. Guimarães, Bernardo R. A. Neves
Department of Physics, Federal University of Minas Gerais, 'Gleb Wataghin' Institute of Physics, Unicamp
In recent years, two-dimensional materials have attracted a lot of attention due to their unusual and novel properties: remarkable charge transport, enhanced optical, electrical, and mechanical properties, as well as other potential applications.
Molybdenum disulfide (MoS$_2$) is part of the family of transition metal dichalcogenides (TMDC), it has a large direct gap in monolayer form. Therefore, MoS$_2$ is a good candidate for optoelectronic applications using 2D materials. When MoS$_2$ is put on top of SiO$_2$, the photoluminescence (PL) of excitons is diminished by the high amount of trions that are also generated. For practical applications, therefore, it is crucial to improve its optical properties by addressing the issue of trions. The method we propose in this study is to use Local Anodic Oxidation (LAO), a standard Atomic Force Microscopy (AFM) technique, to oxidize regions of a MoS$_2$ flake with very high spacial accuracy. If the flake consists of a few layers of MoS$_2$, it is possible to control LAO parameters in order to create MoS$_2$ islands buried in a matrix of its own oxide, MoO$_3$. These can consist of single layers of MoS$_2$ protected by MoO$_3$, thus creating a MoS$_2$-MoO$_3$ heterostructure that inhibits the formation of trions, increasing the PL due to excitons. This results in an effective blue shift of the emission spectrum, a signature which was experimentally confirmed by micro-PL measurements of our fabricated buried islands, which also presented an increase in emission of one order of magnitude relative to non-oxidized regions. The presence of MoS$_2$ under the oxide is further confirmed by detection of its Raman signature. This creates a novel route of fabrication these devices with nanometer precision; the buried island can be as small as dozens of nanometers, and could enable the fabrication of structures that are able to confine excitons in-plane, eventually leading to deterministically fabricated quantum dots in a 2D TMDC.
 
ID 9
Band hybridization effects in GaSb/InAs broken gap heterostructures
Arthur Leão Leutewiler, Marcelo Toloza Sandoval, Tiago de Campos, Leovildo Diago- Cisneros, Guilherme Matos Sipahi 
Instituto de Física da São Carlos - Universidade de São Paulo, Universidad de La Habana-Cuba
Semiconductor heterostructures composed of InAs and GaSb form a junction with broken gap [1]. In this systems, the hybridization of the states from the InAs conduction band and the GaSb valence band is allowed with electrons and holes confined in separated spatial regions, leading to the possibility of gap inversion. As the gap inversion is direct linked to topological properties of such system [2], a better understanding of the processes which allows for the gap inversion is needed. In this work we considered two distinct cases: i) an asymmetric InAs/GaSb quantum well and ii) a symmetric GaSb/InAs/GaSb quantum well. We use the eight band Luttinger-Kohn model together with the envelope function approximation to study the systems, varying the thickness of the InAs layer and how the hybridization occurs. For the asymmetric quantum well, fixing the size of the GaSb layer to 4.8 nm, we observe that there is a critical length of the InAs layer of 9.7 nm where the system undergoes the gap inversion. For the symmetric case, we do not observe the gap inversion, instead there is a critical length of the InAs layer of 10.3 nm were the conduction and valence states become degenerate and the system stays like that. On the other hand, applying a small electric field along the growth direction, we observe an opening of the gap due to the spin-orbit interaction. In addition, for small values of the InAs layer, we see an intriguing energy splitting, which is strongly dependent on the state composition. Summarizing, the symmetry of the confining potential plays a fundamental role on the gap-inversion process, determining the existence of the spin-orbit energy gap which in turn determine the topological properties of the system. [1] H. Kroemer, Physica E 20, 196 (2004). [2] Topological Insulators, edited by M. Franz and L. Molenkamp, Contemporary Concepts of Condensed Matter Science Vol. 6 (Elsevier, Burlington, 2013).
 
ID 10
Spin waves in strained graphene nanoribbons
Jorge H. Correa, Marcos S. Figueira
Universidade Federal Fluminense
Graphene nanoribbons are nanostructures that show interesting electronic and magnetic properties that are special for future spintronics devices, in this sense, we estudy the spin waves in strained graphene nanoribbons. Spin waves are elementaries excitations that occur in magnetic materials, with this in mind, we know that the zigzag nanoribbons  show antiferromagnetic behavior between the opposite edges and ferromagnetic behavior along the same edges, on the other hand, we know that the strain along of  an direction produces deformations in the network of the graphene as mentioned  by [1], the strain is an important effect if we want to open a gap in the network of graphene and open an enormous possibilities if we want properties of the conductance or density of states. To study spin waves in these structures we use the Hubbard's Hamiltonian and it we treat in the RPA and Hartree-Fock approximation, by means of a selfconsistency calculation we can obtain the magnetization values for every site of the ribbon. All the properties that we can study are contained in the transversal magnetic susceptibility in the RPA approximation [2] and is given by $\chi^{RPA} = \frac{\chi^{HF}}{I + U\chi^{HF}}$ with this quantity we can obtain the spectral function given by $A_{ii} = - Imag[\chi_{ii}^{RPA}]$ given us the density of the spin waves, also, we can have the lifetime of these spin waves and relation dispersion in these fascinating structures and to compare these properties with magnetic materials . [1] PRB,80,045401 (2009). [2] New Journal of Physics 13 (2011) 033028
 
ID 11
Wigner entropy production on quantum systems
Franklin Luis dos Santos Rodrigues Junior, Gabriel Teixeira Landi
Universidade de São Paulo
One of thermodynamics most important tasks is to characterize the irreversibility of a process. One of the ways to attempt to do so is via analysis of entropy production. However, a definite formalism for it’s production on quantum systems has not yet been achieved. One of the problems is that phase space methods stumble on quasi-probability distributions only, that is, distributions that do not respect all of the conditions put necessary for a probabilistic distribution. The Wigner one, for example, assumes negative values for some non-classical systems, like entanglement or Fock states. But when Gaussian states are dealt with, it is shown that the Wigner quasi-probability distribution is always positive. This work uses this property to analyze entropy flux and production of diverse systems; namely the harmonic oscillator, optical cavities with dissipation and pump, optomechanical cavities and Dicke’s model on an optical lattice with ultracold rubidium atoms, where the initial state is always Gaussian and is maintained as Gaussian during the dynamics. Due to the experimental relevance of these models, we believe this study will contribute to a better understanding of the irreversibility of a quantum system, with broad applications to modern technological enterprises such as quantum computers, once irreversibility provides a direct measurement of the dissipated resources in an operation.
 
ID 12
Erbium-doping in Zinc Oxide for Photonic Applications
Camila Ianhez Pereira dos Santos, Marcio Peron Franco de Godoy
Universidade Federal de São Carlos
The wide bandgap ZnO (~ 3.37 eV) has a strategic potential in the field of transparent semiconductors. Therefore its transparency in the visible optical range makes ZnO an interesting material to host rare-earth ions. Erbium has a technological appeal for photonic applications due to its optical emissions in the visible and infrared region, which are associated with optical long-distance communications. For an efficient optical emission-absorption in 1530 nm, Erbium ion should be incorporated well-diluted and valence 3+. Particularly, we studied the Erbium doping on ZnO thin films grown by Spray-Pyrolysis technique. This route presents a great versatility in terms of materials choice and allows the preparation of good quality thin films with low financial cost. ZnO:Er thin films were grown in several concentrations up to 4% on glass substrates. The samples were characterized by X-rays Diffraction (XRD), scanning electron microscopy (SEM) and optical transmittance/absorbance measurements. XRD data confirms the growth of polycrystalline wurtzite ZnO:Er thin films with absence of secondary phases and preferentially along the (002) direction. Scherrer-equation analysis shows that an improvement of crystal quality as the concentration of Er increases. Crystallite sizes increase 29Å by % of Er inserted. Another effect observed is the increasing of intensity at peak (002) direction as Er-doping increases independently of sample thickness. SEM data shows that the surface of the films was homogeneous. Optical transmittance/absorbance measurements at room temperature show small influence of Er doping. Photoluminescence measurements were also performed to understand Er-doping in ZnO thin films grown by spray-pyrolysis.
 
ID 13
Accurate deep centers' optical transition energies from DFT-1/2 band structures
Bruno Lucatto, Lucy V. C. Assali, Ronaldo Rodrigues Pela, Marcelo Marques, Lara K. Teles
Instituto Tecnológico de Aeronáutica, Universidade de São Paulo
A major challenge in creating a quantum computer is to find a quantum system that could be used to implement the qubits.
In this scenario, deep point defects in semiconductors (deep centers) are prominent qubit candidates, since most characteristics of their electronic states resembles the ones of single atoms or molecules, with the advantage that they are fixed in space by the surrounding crystal. ab initio calculations are one of the most important tools to theoretically study the properties of deep centers in semiconductors. However, these calculations are highly involved, due to the large supercell needed, and the computational cost can be even larger when one goes beyond the Kohn-Sham scheme to correct the band gap problem and achieve good accuracy. In this work, we present a method that overcomes these problems and provides the optical transition energies as a difference of Kohn-Sham eigenvalues; and even more, provides a complete and accurate band structure of the defect in the semiconductor. [3] The method is an extension of the parameter-free DFT-1/2 approximate quasi-particle correction [1,2] that allows it to be applied in the study of complex defects. As a benchmark, we apply the method to the NV−− center in diamond. The agreement of the results and the experimental data is remarkable, with a accuracy of 0.1 eV. The band structure agrees with the expected qualitative features of this system, and thus provides a good intuitive physical picture by itself. The method has also been applied to study analogues of the NV−− center in materials that are more technologically appealing than diamond, in the search for a defect in a host that enables better integration with microelectronic, photonic and micromechanical structures. [1] L.G. Ferreira, M. Marques, and L.K. Teles - Phys.Rev.B 78, 125116 (2008) [2] L.G. Ferreira, M. Marques, and L.K. Teles - AIP Advances 1, 032119 (2011) [3] B. Lucatto et al. arXiv:1705.10644. Submitted to periodical.
 
ID 14
Protected Helical Edge States in non-topological cylindrical quantum dots
Denis R. Candido, J. Carlos Egues, M. E. Flatté
Instituto de Fisica de São Carlos - Universidade de São Paulo, Department of Physics and Astronomy and Optical Science and Technology Center - University of Iowa
The Bernevig-Hughes-Zhang (BHZ) effective model describes the low-energy physics of a HgTe/CdTe quantum well, the first 2D Topological Insulator (TI) realized experimentally in nature. Here, we solve analytically the BHZ model confined by a cylindrical hard wall using the modified Bessel functions. By solving a transcendental equation following from the appropriate boundary conditions to the problem, we determine all the energies of the system. The wave functions are found to have an analytical closed form, which enhances the fundamental understanding of the origin of the edge states and makes it easier to carry out transport calculations. We show that counter propagating helical edge states are still found in the trivial or non-topological regime in both conduction and valence states. Moreover, they are still protected against elastic backscattering for a large range scale of the quantum dot radius. We have also calculated the circulating currents for system parameters spanning both the trivial and non-trivial regimes, as well as the conductance and density of states through Green's function in linear response. Surprisingly, we show that there is no physical distinction between the feature found in non-topological quantum dot as compared to the quantum dot in the topological regime.
 
ID 15
Porous graphene, graphenylene, porous BN and inorganic graphenylene nanotubes: a theoretical DFT approach
Guilherme da Silva Lopes Fabris, Julio Ricardo Sambrano
LSM/CDMF, São Paulo State University, CEP 17033-360, Bauru, SP, Brazil
The discovery of graphene and later on its inorganic analogues has brought a sudden interest in possible structures generated by these honeycomb-like systems, and it has increased considerably due to its great scientific and technological interest in material science and its possible applications in nanotechnology, as for example, fullerenes, nanoscrolls and nanotubes. However, in its pristine form, graphene is a gapless semiconductor, and this brings restricted application in electronic devices. Nowadays one of the most studied materials are nanotubes, this structure is formed by rolling up a 2D surface, in a specific direction that can be describe by the chiral indices n and m, which can form three possible conformation: armchair (n,n), zigzag (n,0) and chiral (n,m) type. This structure also has unique properties, which depend on the chirality, and besides being possible to apply this material in nanoelectronics it is also possible to use it as a nanofilter and energy storage. Recently a proposal of new porous materials derivated from the graphene and hBN surfaces were published, in which they study porous surfaces named PG (porous graphene) and BPC (biphenylene carbon, or also known as graphenylene.), obtained from graphene, and pBN (porous BN) and IGP (Inorganic graphenylene), obtained from hBN surface, respectively. In this work, in addiction to studying the properties of porous nanotubes generated from the graphene porous surfaces (PG and BPC), we also propose a group of new nanotubes obtained from the BN porous surfaces (pBN and IGP). Based on this, a theoretical study of the properties of PG, BPC, pBN and IGP porous nanotubes is carried out using computational simulations with periodic DFT theory applied to predict the structural, electronic, elastic and topological properties of such porous surfaces. Acknowledgements: This work was supported by the Brazilian Funding Agencies CNPq, CAPES and FAPESP.
 
ID 16
A computational study of the deposition of hexagonal boron nitride into graphene
Julio Ricardo Sambrano, Guilherme da Silva Lopes Fabris
UNESP
Two-dimensional materials are attractive materials with a wide range of applications. Among these, graphene and the hexagonal boron nitride (hBN) has demonstrated your importance in fabrication of electronic devices. Several methods have been developed into engineering band gap to enhance their semiconducting properties. Besides the electronic properties, graphene presents a metallic character and zero band gap, and hBN presents a wide band gap of 5.9eV. Both have similar crystal structure and an mismatch lattice parameters of 1.7%. The computational model of deposition of hBN on a graphene was carried out within the framework of density functional theory applied to periodic methodology, using the CRYSTAL program. This deposition can improve graphene based transistors performance [2]. In this work, are studied and discussed the structural, electronic, elastic and vibrational properties of this disposition. Acknowledgements: This work was supported by the Brazilian Funding Agencies CNPq (46126-4), CAPES (787027/2013, 8881.068492/2014-01) and FAPESP (2013/07296-2, 2016/07476-9). The computational facilities were supported by resources supplied by Molecular Simulations Laboratory, São Paulo State University, Bauru, Brazil. References: Wu, Q.; Wongwiriyapan, W.; Park, J.-H.; Park, S.; Jung, S. J.; Jeong, T.; Lee, S.; Lee, Y. H.; Song, Y. J. Curr. Appl. Phys. 2016, 16 (9), 1175–1191.
 
ID 17
Synthesis and caracterization of cadmium doped ZnO
Ana Laura Curcio, Ariano De Giovanni Rodrigues, Paulo Sérgio Pizani
Federal University of São Carlos
Belonging to the class of wide gap semiconductors, ZnO attracts a great deal of interest for its high spectral selectivity, high electron mobility, high exciton bonding energy and the promising development of ternary alloys, that open possibilities in a wide range of applications in electronic and electrooptical devices such as transistors, photodetectors, light emitting diodes, among others. Its optical emissions are strongly dependent on structural and chemical properties, since defects create levels in bandgap. The controlled insertion of cadmium in the ZnO lattice can be seen as a possible way for tailoring levels in bandgap associated with vacancies or interstitial substitution. ZnO:Cd was produced using two different methods: microwave-assisted solvothermal and mechanical alloying, both presenting as advantages the fast production time, low cost and the possibility of synthesizing nanomaterials. These two types of syntheses essentially give rise to materials with different characteristics in terms of structure and morphology that directly affect the optoelectronic properties. The structural characterizations were performed by the combination of XRD and Raman spectroscopy, through which it was possible to verify that the samples crystallized in hexagonal structure (wurtzite P63mc) without secondary phases. These measures were also important for the investigation of the influence of the synthesis parameters and the proportion of dopant on the structural characteristics. Scanning electron microscopy showed that the produced samples present reduced dimensions (some in the nanometric scale), confirmed by XRD analyses and using a particle size analyzer. Photoluminescence and spectrophotometry measurements enabled us to study the influence of structural characteristics and Cd doping on the emissions due to bandgap transitions and those related to defects and vacancies.
 
ID 18
Optical properties of semiconductor quantum dots with magnetic ions coupled to metallic leads
Helder Faria Andriolo, Pawel Hawrylak
Unicamp, University of Ottawa
We present here the results of the study of a quantum memory encoded in quantum states of a single magnetic ion in a semiconductor quantum dot coupled to metallic leads allowing for control of carrier concentration in a quantum dot. The electronic properties of such a complex system are described as a function of the bias applied to metallic leads, strength of tunnelling matrix element between Fermi sea in the leads and exchange interaction between electrons in a quantum dot and magnetic ions. We analyze here the optical emission process. In the initial state a photoexcited exciton is present. After the excitonic emission, the complete set of electronic states occupation of the magnetic ion couples to the electrons states in the leads. This coupling, necessary for electric manipulation of quantum memory, results in Kondo-like coupling of magnetic spin and electrons in the Fermi sea.
 
ID 19
Theoretical analysis of ZnO/GaN surfaces
Naiara Letícia Marana, Julio Ricardo Sambrano
Modeling and Molecular Simulations Group - São Paulo State University (Unesp) - Bauru
Due to the nonstoichiometry caused by defects such as oxygen vacancies and zinc interstitials, the wurtzite Zinc oxide (ZnO) is an intrinsic n-type semiconductor material. However, from the electronic point of view, pn-type junction materials are promising due to applications in semiconductors devices such as solar cells, light-emitting diodes, photodetectors, rectifier diodes, and bipolar transistors [1]. Based on that, the p-type Gallium nitride (GaN) it is the ideal material to do a heterojunction with ZnO due to its natural p-type, similar semiconductor behavior and slight lattice mismatch (~1.86%).
The ZnO/GaN interface has attracted much attention and they are promising candidates for optoelectronic devices [2]. However, despite the several experimental works about this interface, there are a few theoretical works. Therefore, the aim of this work was to analyze the structural, electronic, elastic, piezoelectric and topological properties of ZnO/GaN (101 ̅0) and (112 ̅0) surfaces. DFT simulations were conducted using CRYSTAL14 program, with B3LYP hybrid functional and all-electron basis set. From the optimized ZnO and GaN bulk, the bare ZnO and GaN (101 ̅0) and (112 ̅0) surfaces, with 24 layers, were obtained and optimized. After the optimization, the inter layers of ZnO and GaN were replaced by GaN and ZnO, respectively, and re-optimized. The results show the greater influence of the outer layer on the structural and electronic properties. The surfaces (101 ̅0) are more stable and presents the lowest surface energy when the ZnO is in the outer layers, as well as the smaller energy band gap. Depending on the material and thickness of the external layers, the observed coloration, in the electromagnetic spectrum, for the ZnO/GaN surfaces changes from UVA to Green. References: [1] U. Ozgur; D. Hofstetter, D.; H. Morkoc, Proc. IEEE, 98, 1255 (2010). [2] K. Maeda; et. al., Journal of the American Chemical Society, 127, 8286 (2005).
 
ID 20
Comparison between bismuth triiodide thin films made by thermal evaporation and solution processes
Natália de Faria Coutinho, Rafael Borges Merlo, Francisco das Chagas Marques
Instituto de Física Gleb Wataghin - UNICAMP
The current world demand for electricity has grown in the last decades, which has led to an opportunity of photovoltaic research to boost. Nowadays, the most promising material to replace the known crystalline silicon solar cell is the lead-halide perovskite solar cell (PSC) that has achieved efficiencies as high as 22.1%. In spite of the incredible efficiency obtained in only few years of research, it faces a problem in stability, apart from having lead, a toxic element. In this way, some lead-free materials suitable for photovoltaic applications have been studied in the last few years, including the bismuth triiodide ($BiI_{3}$)[1], a semiconductor with bandgap of 1,67eV [2]. Usually $BiI_{3}$ is made through solution processes [3], and in this work we compared the $BiI_{3}$ thin films obtained by this route, using a spin coated solution of $BiI_{3}$ in DMF:DMSO, with the thermal evaporated thin films made in an evaporation chamber with pressure of $2 x 10^{-5}$ Torr, using $BiI_{3}$ powder as precursor. We compared the morphology and the crystallinity of $BiI_{3}$ thin films through Scanning Electron Microscope, X-ray Diffraction and Ultraviolet-Visible Spectroscopy measurements. The results indicate that the thermal evaporated thin films are smoother and more crystalline than the spin coating ones, which suggests that the $BiI_{3}$ obtained by physical routes can be a better candidate for photovoltaic applications than the obtained by the currently used solution methods.  Acknowledgements: CNPq, Capes, Fapesp, Lamult Unicamp. References:  [1] Riley E. Brandt et al. J. Phys. Chem. Lett., 6, 4297−4302 (2015). [2] Nikolas J. Podraza et al. Journal of Applied Physics, 114, 033110 (2013). [3] Umar H. Hamdeh. Chem. Mater. 28, 6567−6574 (2016).
 
ID 21
InGaAs/InGaP Multiple Quantum Well Systems for Multijunction Solar Cell for Space Applications
Edgard Winter da Costa, Daniel Neves Micha, Nayara Yohanna Klein, Maurício Pamplona Pires, Patrícia Lustoza de Souza
LabSem/CETUC - Pontifícia Universidade Católica, Grupo de Física Teórica e Experimental - CEFET/RJ, Instituto de Física - Universidade Federal do Rio de Janeiro.
Multijunction solar cells hold the world record efficiency, converting 46% of the solar energy into electricity on ground, and are the very basis of spatial mission’s power supply. Multiple quantum well systems are one of the possible solutions to overcome a major technological challenge of this type of device, namely, finding materials with appropriate bandgap and lattice parameter that can lead to current matching between the stacked pn junctions. In this work, as an alternative to face this issue, we present a theoretical study on the strain-compensated InxGa1-xAs / InyGa1-yP multiple quantum well system to be applied as the active region in such multiple junctions. Some of the simulated quantum well system configurations match the optimal energies for devices containing three junctions for spatial applications. By solving the Schrödinger equation under the effective mass approximation, we were able to map the effective bandgap of the proposed QW system as a function of the thicknesses and alloy’s composition of the different layers. The exchange of the state-of-art GaAsP for InGaP as the barrier material in the QW system brings up the advantage of mechanical resistance in space due to the high radiation hardness of the InP based materials [1]. We showed that it is possible to use the proposed QW system as the active material for a 3-junction MJSC as the intermediate junction under AM0 spectrum. [1] N. Yamaguchi, A. Tatsuya, S. Ohshima, H. Imaizumi and S. Matsuda, "High-radiation-resistant InGaP, InGaAsP, and InGaAs solar cells for multijuction solar cells," Applied Physics Letters, vol. 79, no. 15, pp. 2399-2401, 2001.
 
ID 22
Investigation of the electronic properties of a graphene-Talc heterostructure
Edrian Mania, Anania B. Alencar, Alisson R. Cadore, Bruno R. Carvalho, Kenji Watanabe, Takashi Taniguchi, Bernardo R. A. Neves, Helio Chacham, Leonardo C. Campos
Universidade Federal de Minas Gerais, Universidade Federal dos Vales do Jaquitinhonha e Mucuri, National Institute for Materials Science
Graphene, an atomically thick layer of carbon atoms arranged in a hexagonal lattice, has attracted a lot of interest in basic and in applied physics. With the advent of h-BN crystals, it was possible to improve graphene device quality and uncover many interesting quantum effects of Dirac fermions. On the other hand, heterostructures prepared using other 2D materials do not lead to considerable improvements of graphene devices quality, but they allow the development of other non linear electronics elements. Here we report a graphene van der Waals heterostructure which is able to spontaneously dope graphene (p-type) up to n ~ 2.2 x 10$^{13}$ cm$^{-2}$ while providing excellent charge mobility (~ 25,000 cm$^{2}$V$^{−1}$s$^{−1}$). Such high quality p-type doping is achieved via deposition of graphene on atomically flat layered talc, a natural and abundant dielectric crystal. Raman investigation shows a preferential charge accumulation on graphene-talc van der Waals heterostructures, which are investigated through the electronic properties of talc/graphene/hBN heterostructure devices. These heterostructures preserve graphene’s good electronic quality, verified by the observation of quantum Hall effect at low magnetic fields (B = 0.4 T) at T = 4.2 K. In order to investigate the physical mechanisms behind graphene-on-talc p-type doping, we performed first-principles calculations of their interface structural and electronic properties. In addition to potentially improving solar cell efficiency, graphene doping via van der Waals stacking is also a promising route towards controlling the band gap opening in bilayer graphene, promoting a steady n or p type doping in graphene and, eventually, providing a new path to access superconducting states in graphene, predicted to exist only at very high doping. Acknowledgements: FAPEMIG, CAPES, CNPQ/MCT, INCT/Nanocarbono, Finep, Petrobras, Rede de Nano-Instrumentação and Pós-graduação em Física da UFMG.
 
ID 23
Influence of gravity on eutectic BiSn samples solidified by the vertical Bridgman method
Rafael Cardoso Toledo, Chen Ying An, Irajá Newton Bandeira
Instituto Nacional de Pesquisas Espaciais - INPE
Composition profiles of the eutectic alloy Bi43Sn57 atomic % grown by normal and inverted vertical Bridgman methods are presented and the study of the solute alloy redistribution is made. The inverted vertical Bridgman method (IVB), where the solidification occurs from the top to the bottom of the melt under a destabilizing thermal gradient, allows the growth of crystals with buoyancy-driven convection different from that of the usual vertical Bridgman (VB) configuration. The scope of this work is to study the influence of the gravity in the convection process. The convection is smaller in samples solidified by VB, with more stable solute and density distribution profiles. In both methods the samples presented dendritic structures plus irregular eutectic structure along its length.
 
ID 24
Lead halide perovskite synthesized from sputtered lead sulphide
Francisco das Chagas Marques, Jose Maria Clemente da Silva Filho, Viktor A. Ermakov
Universidade Estadual de Campinas
In the last few years, research on dye-sensitised devices has been focused on the development of solar cells, based on CH3NH3PbX3 (X = I, Br, Cl) composites with perovskite structure. The deposition of perovskite thin films is usually carried out by solution-based processes using spin-coating techniques that result in the production of high quality films. Solar cells made by this method exceed 20 % efficiency, with the potential for use in large scale production through ink print or screen printing techniques. As an alternative route, perovskite thin films can be deposited through thermal evaporation. Here we propose a new method for the production of CH3NH3PbI3, based on a radio-frequency (rf) -sputtering technique that results in a high reproducibility of the films and is compatible with roll-to-roll processes. We deposited a thin film of lead-sulphide (PbS) and converted it into perovskite by placing the film in an iodine atmosphere, followed by dipping in a solution of methylammonium iodide (CH3NH3I). The conversions to PbI2 and CH3NH3I were confirmed by elemental analyses, absorption, and photoluminescence spectroscopy. Structural properties were revealed by X-ray diffraction and infrared and Raman spectroscopy.
 
ID 25
Formation of structural defects during $Bi_{2}Te_{3}$ epitaxy investigated by a Monte Carlo computational model
Celso Israel Fornari, Gabriel Fornari, Paulo H. de O. Rappl, Eduardo Abramof, Jerônimo dos S. Travelho
Instituto Nacional de Pesquisas Espaciais
Bismuth telluride, in $Bi_{2}Te_{3}$ phase, is an archetype of three-dimensional topological insulator. This material presents topological surface states (TSS), shaped like a Dirac cone, crossing the material band gap [1]. These TSS present linear dispersion, resulting in massless Dirac fermions in the surface with extremely high Fermi velocities and bulk insulator behavior. The massless Dirac fermions possess spin-locked to the momentum and are protected from backscattering due to time reversal symmetry, which open-up several possibilities of applications in spintronics and quantum computing [2]. However, presence of structural defects in this compound changes the chemical potential, resulting in bulk conduction which overwhelms the metallic surface states, hampering these topological states from electrical measurements. By controlling the chemical potential of the sample is possible to tune from p to n, passing through a bulk insulator phase. A truly topological insulator compound must have the Fermi level located inside the material band gap, i.e., crossing only the TSS [3]. In this work, we applied a Monte Carlo epitaxial growth model to study the case of bismuth telluride. By changing the growth conditions in the model, we monitored the formation of structural defects. The computational model was validated to a set of experimental data. The simulation results were able to explain a p-to-n transition that occurs by increasing the substrate temperature in which the epitaxial films are grown. References [1] Y.L. Chen et al., Science 325, 178 (2009) [2] Y. Ando, J. Phys. Soc. Japan. 82, 102001 (2013) [3] K. Hoefer et al., PNAS 111, 14979 (2014)
 
ID 26
Growth and characterization of Mn-doped ZnO thin films
Camila Ianhez Pereira dos Santos, Marcio Peron Franco de Godoy
Universidade Federal de São Carlos
Zinc Oxide (ZnO) is a very interesting material due to its wide bandgap (3.37eV) suitable to transparent optoelectronic devices. Manganese (Mn) incorporation in wide bandgap semiconductors is a current topic due to new magnetic and optical properties. Many methods can be applied to obtain Mn-doped ZnO. This work shows an investigation of ZnO thin films doped with Mn at different concentrations (up to 10%) as well two reference-samples: Mn3O4 and ZnO. The samples were grown by Spray Pyrolysis on top of glass substrates using as precursors zinc acetate dihydrate and manganese acetate tetrahydrate in aqueous solution with 10-2 molarity. The system was characterized by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Optical Transmittance and Absorbance. ZnO and Mn3O4 were identified by XRD as wurtzite phase (grown preferentially along the (002) direction) and hausmannite phase (grown preferentially along the (121) direction), respectively. The crystallite size in the sample of ZnO was approximately 13 nm. This size decreased a quarter from 3% Mn-doping with no evidence of secondary Mn-related phases. There was no significant change in the optical gap value up to 7% Mn. Half of the samples was subjected to an annealing at 500°C during one hour. XRD analysis indicated a phase transition from Mn3O4 to alpha-Mn2O3 in the bixbiite phase without a preferential growth direction, and a crystallite size around 25 nm. The images obtained by SEM for as-grown and annealed samples showed Mn segregation after thermal treatment. The bandgap decreased above 1% Mn inserted in the lattice, which corroborates the Mn-segregation for the annealed samples. Our results indicate that the Spray Pyrolysis method is a convenient route to obtain well diluted ZnMnO thin films.
 
ID 27
Strained Graphene in the quantum Hall regime
Daiara Faria, Carlos León, Ramon Carrillo-Bastos, Andrea Latgé, Nancy Sandler
Universidade do Estado do Rio de Janeiro, Universidade Federal Fluminense, Universidad Autónoma de Baja California, Ohio University
The coupling between electronic and mechanical properties in bidimensional materials has become an useful tool to control their properties. Graphene zigzag nanoribbons with longitudinal deformations have been proposed as electronic waveguides for valley polarized currents with quasiballistic properties [1]. Because strained-folds can be easily engineered on graphene samples [2], we study their effects on a graphene membrane in the quantum Hall regime, with an external magnetic field applied to the membrane. The quantum Hall regime has been largely explored in graphene. Therefore the modifications due to the deformation are straightforwardly identified in this regime. A continuum model description allows to obtain analytic expressions for the corrections to the Landau levels and to the pseudospinor eigenstates. A numerical tight-binding band structure calculation for a zigzag nanoribbon confirms these results. Recursive Green's function methods are used to obtain conductance results. The role of valley polarized currents is described and we discuss possible experimental realizations where these effects can be observed. [1] R. Carrillo, C. León, D. Faria, A. Latgé, E. Y. Andrei e N. Sandler, Phys. Rev. B 94, 125422 (2016). [2] E. Y. Andrei et al., arXiv:1703.00550.
 
ID 28
Assessing the electronic states of InAsP/GaAs self-assembled quantum dots by photolumincescence and modulation spectroscopy
Rafaela Moos, Igor Konieczniak, Graciely Elias dos Santos, Ângelo Luiz Gobbi, Ayrton André Bernussi, Wilson Carvalho Jr., Gilberto Medeiros Ribeiro, Evaldo Ribeiro
Universidade Federal do Paraná, LNNano, Lubbock University, BR Photonics, Universidade Federal de Minas Gerais
Semiconductor self-organized quantum dots (QDs) are one of the best candidates for performing tasks such as tailoring the emission energy, band alignment and g-factor. That’s one of the reasons for QD research being still productive nowadays. As the ternary InAsP alloy allow the manipulation of the three physical quantities cited above, it might be a very compelling material. In this work we study, using photoluminescence (PL) and photomodulated transmission experiments, the eletronic states of InAsP/GaAs QDs as a function of temperature and excitation power. PL measurements were performed from 15 K to 290 K, with the excitation power varying from 1 to 20 mW. Some PL experiments were done under linear polarization to assess possible shape anisotropy. We were able to identify the recombination from the QDs ground state and the contribution of the InAsP wetting layer (WL) for temperatures above 100 K, which has never been observed for these QDs. To confirm the presence of the WL, two samples (InAsP QDs and InP QDs) were annealed at 530 K for 1 hour, dissolving the QDs due to an interdiffusion process with the WL. PL measurements on these two samples allowed us to clearly identify the WL electronic states. Specifically, for the InP material, WL recombination is so close in energy to the QD transition that it is possible that both are seen simultaneously as a single PL band. Also, for the InAsP samples with higher As contents excited QD states were identified, since the confining potential is deeper in this situation. Regarding the polarized PL, the InAsP QDs presented shape anisotropy in the [011] direction, revealing to have a slightly elliptical morphology. The WL presents an isotropic behavior, as expected. Modulated transmission shows spectral structures with different ranges of line widths, which are consistent with the QD and WL assignments given after the PL analysis. Authors acknowledge financial support from CNPq, CAPES, and Fundação Araucária.
 
ID 29
An optical and morphological analysis of CeO2 nanowires obtained by electrospinning
Evaldo Ribeiro, Renato Fernando Caron, Bruno Morais Serafim, Ney Mattoso, Cyro Ketzer Saul
Universidade Federal do Paraná
Cerium dioxide is known for its wide range of technological applications, the most known being its use in cathalysis. For this application, particularly, a large surface-to-volume ratio would greatly improve the cathalitic process efficiency. The aim of this work is to produce CeO2 nanowires from different precursors, such as cerium acetate, and then study the electronic states of the wires as a function of size and morphology. The fiber production is based on a cerium acetate, polyvinyl alcohol and sodium lauryl sulfate solution in water, which was deposited via electrospinning on a metallic target under 15 kV acceleration voltage, giving rise to a polymeric membrane, showing the formation of fibers. The sample was fractioned and each piece was annealed at 350, 450, 550, 650, 750, 850, and 950 oC, for 30 minutes. Scanning electron microscopy has identified the formation of the polymer fibers and has confirmed that wires of nanometric diameters were obtained after thermal treatment. Raman scattering confirmed the formation of CeO2 material, which has been further indicated by X-Ray diffraction. In both techniques the line widths of the characteristic spectral peaks shows a decrease as a function of increasing annealing temperature, indicating that the nanowire cristalline coherence length does increase for increasing treatment temperatures. Scanning electron microscopy revealed that wire diameters also decrease (from 300 nm to 50 nm) as the annealing temperature is increased. In previous works on ceria films we found that higher annealing temperatures give rise to an increase of defects that translated into the emission spectrum as green- and red-centerer defect bands. Photoluminescence experiments are being carried at the present time in order to verify if this would be the case for the CeO2 nanowires. Authors acknowledge financial support from CNPq, CAPES, and Fundação Araucária. Raman measurements were performed at the Centro de Microscopia Eletrônica, UFPR.
 
ID 30
Density functional theory calculations of functional groups on bismuthene
Erika Nascimento Lima, Andreia Luisa da Rosa, Renato Borges Pontes, Dominike Pacine de Andrade, Thomas Frauenheim
Universidade Federal de Mato Grosso, Universidade Federal de Goiás, Instituto Federal de Goiás, Bremen University
Quantum spin Hall (QSH) insulators feature edge states that topologically protected from backscattering. However, the major obstacles to application for QSH effect are the lack of suitable QSH insulators with a large bulk gap. Currently, to solve this problem the functionalization of small molecules has been widely used to improve stability and the nontrivial band gap of new 2D films. We use density-functional theory calculations to investigate the electronic properties and topological insulating characteristics of bismuthene functionalized with functional groups containing hydrogen. In particular, we show that methyl-functionalized bismuthene is a 2D topological insulator with protected Dirac type topological helical edge states. Furthermore, the nontrivial topological characteristic of this system is confirmed through the calculation of the Z2 topological invariant, showing that hydrogen functionalized bismuthene is promising for applications in quantum spin Hall devices.
 
ID 31
In/Cd impurities in GaN and AlN
Helena Maria Petrilli, Lucy V. C. Assali, José Mestnik Filho(in memorian), Stefaan Cottenier, Reiner Vianden
Universidade de São Paulo - Instituto de Fisica, Instituto de Pesquisas Energéticas e Nucleares, Ghent University, Bonn University
Results of electric field gradients (EFG) acting on 111In/111Cd probes implanted into GaN and AlN crystals, measured by means of perturbed angular correlation (PAC), are compared with results of first-principles electronic structure calculations in which several crystallographic sites and charge states of the defects were tested. In GaN, two PAC signals were detected. Comparison with the calculations allow us to ascribe them both to In/Cd atoms substituting for Ga positions but with two different charge states: at low temperatures, 65% of cadmium impurities present a negatively charged (-1) state while the remaining 35% are neutral. At temperatures above ≈500 K, all probes have the negatively (-1) charged state. In AlN, the situation is more complex since three PAC signals were detected. Two of them, with relatively small EFG’s, can also be ascribed to In/Cd atoms replacing for Al in two distinct charge states as in the GaN case. The third signal however, with a large EFG, could only be ascribed to In/Cd atoms sitting at an Al site presenting also a N vacancy. This defect has a positively charged (+1) state.
 
ID 32
Weak localization in graphene with disordered ripples
Caio Lewenkopf, Tatiane Pereira dos Santos, Leandro Lima, Rhonald Burgos
Universidade Federal Fluminense
We study the effects of ripples in the magneto-conductance of disordered bulk graphene. Typical lattice deformations whose characteristic lengths are much larger than the lattice parameter smoothly change the interatomic distance. The latter affect the local electronic structure and can be mapped into an effective pseudo-magnetic vector potential whose magnitude scales with lattice distortions. Ripple disorder gives rise to random magnetic fields that affect the electron mobility and suppress the quantum mesoscopic interference effects, such as weak localization. We approach this problem both numerically and analytically. For the numerical disorder simulations we use a tight-binding Hamiltonian. The conductivity is calculated using the Landauer approach. Hence, ripple disorder is modeled by properly accounting for the hopping integrals between neighboring sites of a smoothly deformed lattice. In addition the model includes short-range scattering, ubiquitous in graphene systems. We show that these types of disorder determine the sign of the quantum corrections to the conductivity at low external magnetic fields making the magneto-conductivity transit between a weak-localization (WL) and an weak-antilocalization profile (WAL) as function of the disorder strength and range. We also consider an effective Dirac Hamiltonian and use the diagrammatic approach to study competition between the different sources of disorder and support our interpretations.
 
ID 33
Spin relaxation in graphene with vacancies
Vladimir Gonçalves Miranda, Eduardo R. Mucciolo, Caio H. Lewenkopf
Universidade Federal Fluminense, University of Central Florida
Due to its large electron mobility and a low intrinsic spin orbit interaction, Graphene has been proposed as a promising material to be applied in spintronics. A decade has elapsed since the first successful experimental demonstration of spin current injection and manipulation in graphene. A puzzle has also emerged since this date: a three-orders of magnitude difference between theoretical predictions and experimental measurements of the spin relaxation times. A lot of theoretical and experimental efforts have been devoted to understand and solve this puzzle. Different mechanisms of spin relaxation have been proposed. One of the most important relies on magnetic impurities that would act as "spin hot spots" flipping the spins of the electrons as they are scattered. Theoretical models and experiments have shown that vacancies and hydrogen adatoms can induce $\pi$-like magnetism in graphene. This magnetism is very interesting and unusual since usually magnetism is related to atoms with d and f unfilled shells and is very localized around these atoms. However, $\pi$-like magnetism in graphene involves only p electrons in the outer shell and leads to a spin texture that extends over many sites around the defect. The main difficulty in building a model to study spin relaxation due to hydrogen adatoms or vacancies in graphene is the "extended" nature of the induced $\pi$-like magnetic textures and none of existing models in the literature deals with this behavior properly. A recent study shows that close to the Dirac point the energy dependence of spin relaxation times estimates have a strong dependence on the range of the magnetic impurities. In this work we address this issue and derive an impurity model that takes into account the effect of puddles and magnetic impurities in graphene allowing the incorporation of the spin texture related to the defect-induced state with its spatial extend and modulations along the lattice sites. 
 
ID 34
Electroluminescence and Electronic Transport of Resonant Tunneling Diodes depending on Magnetic Field and Temperature
Andrea Naranjo Lopez, M. D. Teodoro, E. R. Cardozo, V. L. Richard, G. E. Marques, L. K. Castelano, A. Pfenning, F. Hartmann, S. Höfling
Universidade Federal de São Carlos, Wuerzburg University
Resonant Tunneling Diodes (RTDs) are a type of semiconductor dispositive widely used in microelectronics and also, for understanding some quantum physics phenomena. One of the most interesting phenomenon these structures presents is bistability, where the current flows in different trajectories going back and forth in terms of applied voltage, and may be originated from both intrinsic properties and extrinsic system parameters. In this work, we studied the extrinsic bistability of a GaAs/AlGaAs standard RTDs, due to an external resistance associated in series to the diode. The characteristic current vs. voltage curves (I x V) and the Electroluminescence (EL) spectra as a voltage function were obtained for different magnetic fields. It was possible to identify, from the obtained results of optical measurements, four emission peaks along the spectrum: two peaks associated with Galium Arsenide emission, one related to the quantum well, and the last one related to Aluminium Galium Arsenide layers. The electrical current variation dependent on the applied voltage shows a clear bistability region in the presence of associated resistor, allowing an electrical hysteresis behavior, light emitting increase of several orders of magnitude and access to higher voltage regions, which would not be reached without the use of the resistor. In addition, it was found that the bistability area is strongly modulated by the magnitude of the resistance. We continue to study the applied magnetic field influence to spin properties from both electric and optical points of view, with and without the association of external resistance.
 
ID 35
$Zn_{1-x}Cu_xO$ Thin Films Grown by Spray-Pyrolysis Technique
Diego Scolfaro da Silva, Marcio Peron Franco de Godoy, Ariano de Giovanni Rodrigues
Universidade Federal de São Carlos
Transparent conducting oxides (TCO) like Zinc and Copper Oxides are a promising solution for device development and energy generation due to their physical properties and versatility of applications, such as solar cells, light emitters, media storage devices, catalysis and battery electrodes. Besides low-cost, phase-stability and nontoxicity, attractive properties can be explored in a ZnCuO ternary alloy or solid solution which include the bandgap engineering from visible to ultraviolet and transitions between direct-indirect band gap as well as p-type to n-type semiconductor. In the present work, Zinc oxide (ZnO), Copper oxide II (CuO) and the $Zn_{1-x}Cu_xO$ alloy thin films were grown on top of glass substrates by Spray Pyrolysis Technique (SPT). Our chemical route employs Zinc and Copper di-hidratated acetate as precursor in water solutions with molarity $4*10^{-3}$ in the temperature range of 220°C – 300°C. X-Ray Diffraction (XRD) reveals the growth of ZnO in wurtzite phase with preferential orientation [002] and mean crystallite size 135 Å. CuO presents a monoclinic structure (special group C2/C) with preferential orientation [002] and mean crystallite size 145 Å. Phase segregation is observed for the alloys when copper concentration is higher than 0.25, followed by a decrease in crystallite mean size. Optical transmittance shows 3.28 eV absorption energy for ZnO and 1.91 eV for CuO, being both direct allowed transitions. Reduction of the near absorption edge as a result of increasing copper concentration is also observed. Photoluminescence studies reveals presence of defect-related emissions for low copper concentrations.
 
ID 36
Spectral shift of TMPyP Porphyrin/Clay interface
Eduardo Diaz Suarez, A. V. Gil Rebaza, Filipe Dalmatti Lima, Vera R. L. Constantino, Helena M. Petrilli
Instituto de Física - Universidade de São Paulo, Facultad de Ciências Exactas - Universidad Nacional de La Plata-UNLP, Instituto Federal de Educação Ciência e Tecnologia de São Paulo, Departamento de Química Fundamental - Instituto de Química - Universidade de São Paulo.
The electronic spectra shift of tetracationic 5,10,15,20-tetrakis(1-methyl4-pyridyl)-21H,23H-porphyrin (TMPyP) upon adsorption on a clay surface is here investigated using ab-initio electronic structure calculations performed in the framework of the Density Functional Theory (DFT). Porphyrins are heterocyclic macrocycle organic compounds with many applications such as photosensitizers for light harvesting and chemical reactions, molecular electronics and enzymatic catalysis. They can be found in biological systems like photosynthesis of light, enzymes, and transport proteins. The porphyrins optical spectra can be characterized by the presence of a dominant so called Soret band plus a Q-band structure, whose positions and shapes offer a method to characterize porphyrins in various environments. The influences of the environment and DFT exchange-correlation functional on the UV-vis spectra of the model systems, here considered, are investigated. With this aim, the TmPyP/Montmorillonite interface is studied and these results are further discussed to tentative address the observed electronic spectral shift on TMPyP upon the TMPyP/Clay interaction. The results suggest that the mechanism is still controversial.
 
ID 37
Aqueous synthesis and optical propierties in CdSe/CdTe and CdSe/CdTe heteroestructures.
M.A.G Balanta, V. C. Solano-Reynoso, R. F. Cuevas
Universidade Federal de Uberlândia-FACIP, MG-Brasil, UNESP - Campus de Ilha Solteira - DFQ, SP-Brasil, Universidade Federal de Uberlândia-FACIP, MG-Brasil.
The synthesis of semiconductor colloidal nanocrystal (NCs) have attracted attention due to their great potentials in sensors, bioimages, biomedical tags and so forth. In this work, we have investigated the process and optical properties of the CdTe/CdSe core-shell semiconductor nanocrystal synthesized in aqueous colloidal solution. The CdTe core were first synthesized in water by injecting freshly prepared NaHTe solution into N2 -saturated CdCl2 .2H2O and (TGA) thioglycolic acid dissolved in ultrapure water at pH=13,7. The resulting mixture was then refluxed under N2 to promote the core growth to the desired size. In this experiment, the CdTe cores have approximately 3,76 nm in diameter. For CdSe shell synthesis: The CdTe cores were dispersed in 15 mL solution of CdCl2.2H2O and TGA diluted in 20 mL of ultrapure water under N2 and heated at 55oC by 30 min. Then 2 mL of freshly prepared oxygen-free NaHSe solution was injected rapidly under vigorous stirring. The reaction mixture was then refluxed until the completion of the shell at the desired shell thickness. Aliquots of the sample were taken at 60, 90, 210, 270 min and used to record their optical properties. The UV-Vis absorption spectra were measured at room temperature in range from 300 to 700 nm. The absorption peaks were observed between 526-542 nm. . The confinement effect was reinforce with cyclic voltammetry (CV) measurements. Both photoluminescence and CV measurmentes revealed that CdTe/CdSe nanocrystals size increasing when the refluxing time increased from 2,86 nm for CdTe core to 3,08 nm for sample refluxing by 270 min. The shell thickness increasing from 0,9 to 1,2 nm. The surface quality is also studied from both techniques, and our results showed a significant improvement surface quality as danling bond are eliminated by increasing the CdSe Shell thickness. [1] Anne S. Schulze et al,. Journal of Nanoparticle Research (2017) 19:70.
 
ID 38
Magneto-photoluminescence studies of MoSe2/hBN heterostructures
Felipe Soares Covre, Vanessa Orsi Gordo, Odilon Divino Damasceno Couto Júnior, Fernando Iikawa, Freddie Withers, Jorlandio Francisco Felix, Yara Galvão Gobato
Departamento de Física - Universidade Federal de São Carlos, Instituto de Física Gleb Wataghin - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin - Universidade Estadual de Campinas, Quantum Systems and Nanomaterials group - School of Physics and Astronomy - CEMPS - University of Exeter, Universidade de Brasília - Instituto de Física - Núcleo de Física Aplicada
Monolayer transition metal dichalcogenides (TMDs) have recently emerged as an interesting semiconductor material in which coupled spin-valley physics can be explored. They exhibit a direct band gap located at two inequivalent ± K valleys. In addition, TMDs give rise to interesting new phenomena in external magnetic fields such as valley Zeeman splitting and magnetic field induced valley polarization. In this work, we present our preliminarily results of the polarization resolved magneto-photoluminescence (PL) of a monolayer MoSe2 crystal prepared by mechanical exfoliation and transferred onto a hexagonal boron nitride (hBN) crystal. The optical measurements were performed by using a 532nm linearly polarized laser under magnetic fields up to 15T perpendicular to the monolayer (Faraday configuration) at T= 10 K. Two emission bands were observed at lower temperatures. The lower energy emission around 1.620 eV is attributed to the charged exciton (trion) recombination and the higher energy band 1.648 eV was attributed to the neutral exciton (X0) emission at 10K. We observed a clear Zeeman splitting for both the neutral exciton and the charged exciton. The extracted Landé-factors for exciton (g) is found to be approximately -1.1. The degree of circular polarization increases with increasing magnetic field for both emission bands even when using linearly polarized excitation. For exciton emission, the polarization degree is around 10% while for trion emission is about 16% at 15T. The magneto-induced circular polarization seems to result from the preferential occupation of the lowest Zeeman state of the exciton or trion. Ref: A. V. Stier, K.M. McCreary, B.T. Jonker, J. Kono, S.A. Crooker, Nature Communications, DOI: 10.1038/ncomms10643 (2016)
 
ID 39
Direct Evidence of traps controlling electronic transport properties of SnO2 nanobelts
Olivia Maria Berengue, Adenilson José Chiquito, Rosana Alves Gonçalves
São Paulo State University (UNESP) - Faculty of Engineering - Campus Guaratinguetá, Federal University of São Carlos
Quasi 1D metal oxide nanostructures have attracted considerable interest in the last years for fundamental studies and also for potential applications. In particular, they present properties which ranges from metals to semiconductors and insulators. One of the most prominent application of these materials is in the gas sensing devices. SnO2 is an intrinsic n-type semiconductor (mostly induced by oxygen vacancies) usually found in a rutile structure. The coexistence of low resistivity and optical transparency is a characteristic feature of this class of materials but the electrical conductivity is unstable due to the reaction of oxygen vacancies in the SnO2 and the environment atmosphere. In fact, this feature makes SnO2 an excellent material form gas sensing devices either as thin films or as nanowires and nanobelts This work reports on direct evidences of localized states in undoped SnO2 nanobelts synthesized from vapour-solid mechanism. From current-voltage measurements in backto-back Schottky diodes we found a temperature dependence of the barrier height. This is a signature of charges localization at the metal-semiconductor interfaces and in regions nearby. Also, hopping transport was identified in broad range of temperatures (low temperatures) while Arrhenius excitation was observed in high temperatures. The strength of electron's localization was studied by thermally stimulated current measurements (TSC). This method is broadly used for determining trapping parameters in semiconductors. We found two energy levels which are in agreement with the hopping and activation mechanisms observation. This data can provide new insights to understand the performance of sensors or transducers based on these samples.
 
ID 40
Optical Properties of p-doped Dilute Nitride Semiconductors Under Electric Fields
Sara C. P. Rodrigues, Thiago F. de Oliveira, Guilherme M. Sipahi
Departamento de Física, Universidade Federal Rural de Pernambuco, Instituto de Física - Universidade de São Paulo
InGaAsN is a candidate material to realize the ultrahigh efficiency lattice-matched multi-junction solar cell, photovoltaic power source for communications satellites and fiber-optic lasers. This material has the 1eV band gap energy and same lattice constant as GaAs or Ge substrate by controlling the In and N compositions to be 9$\%$  and 3$\%$ , respectively. So far, the highest conversion efficiency of the lattice-matched multi-junction solar cell is 43.5$\%$  which was achieved by the 3-junction solar cell, GaInP/GaAs/GaInNAs. However, despite their great potential for applications, the understanding of their physical properties is rather incomplete. In particular, the dominant mechanisms of light emission in these alloys and their dependence on the nitrogen composition are not well established. Such information is crucial not only for a better understanding of the optical properties of the nitrogen containing III-V alloys, but also for a better technological control of alloy formation and optimization light emission efficiency. Another point concerns to investigation in p-type doping in InGaAsN. This is of great importance since, for example, can improve the transport in HBT (Heterojunction Bipolar Transistors) devices. In this work we report on theoretical luminescence spectra calculations for p-doped GaAs/InGaAsN quantum wells and superlattices. The calculations are performed within the $\vec{k}\cdot\vec{p}$ method by solving the full 8 $\times$ 8 Kane Hamiltonian, generalized to treat different materials. Strain effects due the lattice mismatch between InGaAsN and GaAs are taken into account. By varying the acceptor concentration we analyze the effect of exchange-correlation, which plays an important role in profile potential and electronic transition. These results can explain several important aspects about optical properties in these systems.
 
ID 41
Hybrid density-functional calculations of formic acid on anatase TiO2 (101) surfaces
Andreia Luisa da Rosa, Erika Nascimento Lima, Liangzhi Kou, Thomas Frauenheim
Universidade Federal de Goias, Universidade Federal de Mato Grosso, University of Bremen
There is a considerable technological interest in combining organic and inorganic semiconductor materials in order to obtain hybrid materials with novel properties. One of the crucial aspects for the realization of these hybrid systems is the understanding of the semiconductor/organic interface. Molecules containing carboxyl groups (-COOH) are the most used anchor group on TiO2, both on rutile and anatase phases. The role of this anchor group is many fold: they can assist charge tranfer at the interface, help to stabilize the oxide surfaces against ecthing and provide a strategy for further immobilization of dyes and biomolecules. Contrary to the rutile phase, it is well known that anatase (101) surfaces are not easily reducible, as it has been reported in several experimental and theoretical investigations. On these surfaces, the barrier for surface diffusion towards the deeper layers should allow oxygen vacancies to reside at subsurface sites. Recent vibrational spectroscopy investigations have suggested that acetic acid adsorbs in a bidentate configuration in the presence of oxygen vacancies, whereas a monodentate mode was found on perfect anatase TiO2 (101) surfaces. How the presence of oxygen vacancies in both surface and subsurface sites affects the adsorption of molecules containing -COOH groups on reduced anatase TiO2 surfaces has been investigated. In this work, density functional theory (DFT) calculations using the Heyd-Scuseria-Ernzenhof form (HSE06) for the exchange correlation functional has been employed to clarify the microscopic nature of the interaction of formic acid with perfect and defective TiO2 (101) surfaces at low coverages. We demonstrate that acetic acid adsorbs in a monodentate mode on both perfect and defective surfaces. Moreover, the presence of oxygen vacancies affects the adsorption geometry, but not the adsorption mode, which helps to clarify recent experimental observations using vibrational spectroscopy.
 
ID 42
PbSnTe surface states in the envelope-function approximation 
Erasmo A. de Andrada e Silva, Giuseppe C. La Rocca
Instituto Nacional de Pesquisas Espaciais - INPE, Scuola Normale Superiore - SNS - Pisa
We investigate theoretically the surface states of the topological insulator (TI) $Pb_{1-x}Sn_xTe$ compound within the envelope-function approximation. Heterostructures with inverted and large-gap materials are used to model the surfaces of both bulk and thin films [1]. Spin-orbit (SO) interaction is considered within the Dimmock kp model for the lead-salts, analogous to the Kane model for the III-V compounds. With standard envelope-function procedure, employing spin-dependent boundary conditions, and for the main surface directions (i.e., [111], [100] and [110]) we calculate the dispersion relation of the band of surface states laying in the PbSnTe bulk band-gap. The dependence with the thin-film thickness and with different models for the interface with the vacuum is studied. It is shown that simple expressions describing the typical TI Dirac-cone structure, with SO momentum-spin locking, are in this way easily obtained. For thin films, the opening of the Dirac-cone gap [2] and the Rashba-splitting of the surface states is clearly reproduced, and the surface-direction dependence discussed. In conclusion, the model calculation proposed is shown to be transparent and quantitatively precise enough to be useful in the investigation and in the application of the electronic properties of these TI surface states. [1] R. Buczko and L. Cywinski, Phys. Rev. B 85, 205319 (2012). [2] V. Korenman and H. D. Drew, Phys. Rev. B 35, 6446 (1987).
 
ID 43
Induced ferromagnetism in mechanically milled nanocrystalline In2O3 powder
Y. Galvão Gobato, M. H. Carvalho, M. Pizzo Piton, S.P. Amaral Souto, H.V. Avanço Galeti, O.M. Lemine, M.Bououdina, A. J. A. de Oliveira
UFSCAR, USP-FZEA-ZAB, Al Imam Mohammad Ibn Saud Islamic University
In this work, we have investigated structural, optical and magnetic properties of commercial powder In2O3 semiconductor nanoparticles (NP) (purity: 99.9%) mechanically milled in air using a planetary ball mill with Zirconia grinding medium in order to avoid any contamination with magnetic impurities. We have performed XRD, Raman Spectroscopy, Photoluminescence and SQUID measurements. The XRD results evidences a bcc-phase for In2O3 semiconductor , a reduction of crystallite size and changes of microstrain with increasing milling times. Particularly, it was observed an initial increasing of microstrain followed by a decrease after 24h milling time. The PL spectra has showed a visible region emission related to oxygen vacancy transition. This PL band has showed a blue shift with increasing milled times due to quantum confinement effects. Raman peaks have showed a red shift and a broading with increasing milling times associated to oxygen vacancies, changes of strain and/or phonon confinement. In addition, it was observed that the milled samples have evidenced a ferromagnetism order at room temperature. The coercive field as function of milling time has shown a good correlation to microstrain with a maximum value for the 24 hours milled sample followed by a decrease with increasing milling time. Finaly, our results evidence that the microstrain plays a crucial role in the ferromagnetic behavior of In2O3 semiconductor nanoparticles.
 
ID 44
Synthesis and Characterization of Nanobelts and Microbelts of $Sb_{2}O_{3}$
Danilo Fernandes da Silva, Olivia Maria Berengue, Rosana Alves Gonçalves
Universidade Estadual Paulista - Campus Guaratinguetá
Along the development of nanotechnology and scale reduction of semiconductor materials, a great importance is given to nanowires and nanobelts through material physics studies, also as strong candidates to integrate optical-electronical nanostructured devices. Within the nanostructured materials group we can quote TCO - Transparent Conductive Oxides as a class of important materials for this kind of application. One of them is the Antimony Oxide ($Sb_{2}O_{3}$). For this work, it was synthesized nano and microstructures (nanoparticles, nanobelts and microbelts) of antimony oxide through vapor-solid (VS) growth method associated with a process of carbothermical reduction seeking to reduce the temperatures used in the synthesis process. Once we got the samples, these were characterized through techniques such as x-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman Spectroscopy looking to analyze the crystalline sctructure and quality, morphology and possible preferential growth regions on the synthetised samples. The SEM analysis showed us our samples are synthesised by the VS method and they have belt-like geometry. The XRD analysis showed us two different phases of $Sb_{2}O_{3}$, one of them cubic and another orthorhombic. We have found that synthesised nanoparticles rather grow in cubic phase with high crystal quality. The orthorhombic phase appears in nanowires and nanobelts even when there is a cubic phase too where it was obtained evidences that exists a preferential growth direction [$222$] (Punctual group Fd-3m, PDF $01-072-1334$). The Raman spectroscopy showed us not only a monocrystal character of the samples such as selection rules which could be associated with the presence of structural disorder in our samples. This disorder could be explained if we consider the presence of intrinsic defects within these oxides such as oxygen vacancies. 
 
ID 45
Comparison between infrared photodetectors based on submonolayer quantum dots(SMLQDs)and on usual Stranski-Krastanov quantum dots (SKQDs).
A. Alzeidan, M.S. Claro, A.A. Quivy
Universidade de São Paulo - Instituto de Física
InAs quantum dots are generally fabricated on GaAs substrates using the Stranski-Krastanov growth mode, which consists of the deposition of a thin and strained InAs layer that, above a thickness of 1.7 monolayers (MLs), partially relaxes to form small InAs islands that behave as quantum dots (QDs). Since the size of such nanostructures can only be modified in a very limited range, an alternative to grow QDsina more flexibleway,with a higher surface density and an improved 3Dquantum confinement, is to use a submonolyer (SML) thechnique. SML-QDs can be obtained by depositinga fraction of a monolayer of InAs material (30-50%) in order to nucleate a high density of small two-dimensional (2D) islands on the GaAs substrate, and then covering these islands with a few MLs of GaAs material. This cycle can be repeated as many times as necessary and, in such conditions, the small 2D InAs islands of adjacent layers will pile up, due to the local strain field, and will form QDs having the desired height. Initially, 2 quantum-dot infrared photodetectors (QDIPs) with a similar structure but containing either usual SKQDs or SMLQDs were grown by MBE and processed in our laboratory using conventional photolithography, wet etching and metallization techniques. However, due to the low responsivity and much higher dark current detected in the SMLQDIP (probably due to the non-optimized doping and much larger QD density), another QDIP was grown containing SMLQDs inserted inside an AlGaAs QW. Since, in such a structure, the transition responsible for the main absorption was due to a bound-to-bound transition between the ground state of the SMLQDs and an excited state of the QW close to the top of the AlGaAs barrier, a much higher responsivity could be achieved, although the dark current was still higher than for a conventional QDIP using SKQDs. All the experimental results and possible improvements will be discussed.
 
ID 46
Study of the Anderson model using the one crossing approximation (OCA)
Bruno Max de Souza Melo, Caio Henrique Lewenkopf, Alexandre Reily Rocha
Universidade Federal Fluminense, Universidade Estadual Paulista Julio de Mesquita Filho
The Anderson impurity model is the “standard model” to describe the behavior of systems where the electronic interaction is important such as strongly interacting quantum dots and doped metals with magnetic impurities. The numerical solution of this model also constitutes an essential step in the calculation of the properties of strongly correlated systems, in the context of the Dynamical Mean Field Theory (DMFT) [1]. The Anderson model can be solved numerically by several methods, which differ mainly by the computational cost involved and by the range of the model parameters where the used method is reliable. In this work we study the one-dimensional Anderson model using two methods: one crossing approximation (OCA) [2] and equations of motion (EOM) [3]. We calculate the Green function of the interacting site and the zero bias transmission coefficient for different values of interaction, temperature and coupling between the site and its neighborhood. We study the coupling in the wide band limit and in the case of a linear chain. We analyze the symmetric case $ (2 \varepsilon _{0}+U=0)$ and the asymmetric case $ ( 2  \varepsilon_{0}+U \neq 0 )$, including the large U limit. Then, we compare the results from both methods. Our goal is to validate the one crossing approximation as a solver for the Anderson impurity model, aiming at a future integration with the DMFT, DFT and NEGF formalisms to study the transport properties of strongly correlated systems [4]. [1] G. Kotliar et al., Rev. Mod. Phys. 7, 865 (2006) [2] C. N. Sposetti et al., Phys. Rev. B 94, 085139 (2016) [3] V. Kashcheyevs et al., Phys. Rev. B 73, 125338 (2006) [4] D. Jacob, J. Phys.: Condens. Matter 27, 245606 (2015)
 
ID 47
Electroluminescence from transition-metal dichalcogenide heterobilayers
David A. Ruiz-Tijerina, Mark Danovich, Vladimir I. Fal’ko,
National Graphene Institute, University of Manchester
Transition-metal dichalcogenides (TMDs) are two-dimensional (2D) direct-gap semiconductors, known for their remarkable electronic and valleytronic properties. Coulomb interactions are enhanced in TMDs due to their low dimensionality, making them fertile ground for the formation of excitons, and thus very attractive for optoelectronic applications. We have studied incommensurate TMD heterobilayers of the type MoX22/WX22, with X=S, Se, whose staggered band alignment leads to the appearance of long-lived interlayer excitons that dominate the optical response. We show that intervalley Coulomb interactions can compensate for the valley mismatch in incommensurate and slightly twisted bilayers, thus enabling in-plane-circularly polarized, and out-of-plane linearly polarized electroluminescence, by means of intermediate electron-electron scattering processes. We discuss the importance of our results in the context of recently observed upconversion electroluminescence in graphene-TMD van der Waals heterostructures.
 
ID 48
Selective area growth modeling based on the vapor-phase diffusion model
Alvaro Diego Bernardino Maia, Guillaume Binet, Pierre-Yves Lagrée, Jean Decobert
Sorbonne Universités - UPMC Univ Paris 06 - CNRS - UMR 7190 - Institut Jean le Rond d’Alembert - Paris - France, Almae technologies - Marcoussis - France, III-V Lab - Marcoussis - France
We report a vapor-phase diffusion model (VPD) applied to the development of a photonic integrated platform based on the Selective Area Grow (SAG) technique by Metalorganic Phase Vapor Epitaxy (MOVPE) growth. The SAG of III-V semiconductors' materials is an important tool to achieve the monolithic integration of active and passive photonic functions of optoelectronic devices, opening the way to the achievement of reduced cost InP-based photonic integrated circuits. In SAG, the growth occurs on a wafer partially covered with a dielectric material mask. As no species can deposit on the amorphous surface of the dielectric mask, a concentration gradient appears in the vapor phase and the active species diffuse from above the masked areas toward the open areas. This concentration gradient leads to an enhanced growth rate in the vicinity of the mask. The enhanced growth rate induces a spatial compositional shift in the case of ternary and quaternary alloys, due to the different decomposition and diffusion rates of group III element' precursors in the vapor phase. This technique allows different band-gap materials be defined simultaneously in a single epitaxial growth, leading to well-controlled and simplified processing in comparison with etch-regrowth-based methods. The model was built in a two-dimensional domain which calculates material thickness across infinite dielectric stripes and determines the effective diffusion lengths of different species corresponding to specific growth conditions. This was achieved by fitting experimental thickness values for simple binary materials. Secondly, the knowledge of effective diffusion lengths was used to develop a three-dimensional model suitable for the simulation of SAG growth around an arbitrary mask shape. In the masks used for the fabrication of the components, a periodic layout is often adopted where each mask pattern is surrounded by identical neighbors that may influence SAG.
 
ID 49
Study of Stability and Electronic Properties of Transition Metal Dichalcogenides using Density Functional Theory
Carlos Maciel O. Bastos, Fernando P. Sabino, Guilherme M. Sipahi, Juarez L. F. Da Silva
IFSC - University of São Paulo, IQSC - University of São Paulo
In contrast with graphene, layered transition-metal dichalcogenides (TMDCs) can show semiconductor, conductor, superconductor and insulator properties. Although all those properties can be obtained, there is no clear recipe yet that provide a route to designing layered TMDCs with particular properties. Therefore, there is a great interest to screen the stability and electronic properties of the layered TMDCs bulks, a requirement to distinguish all possible combinations in the different groups, i.e., semiconductors, conductors, etc. In this work, we report density functional theory (DFT) calculations with van der Waals corrections for nine different TMCD compounds, in two crystalline phases, combining Zr, Nb, and Mo with S, Se, and Te. We initially addressed the stability of the compounds, finding that Zr compounds prefer to crystallize in the phase 1-T, Mo in the phase 2-H and Nb in both phases, the latter evidenced by the small energy difference between the two phases. This behavior is observed experimentally and depends predominantly on the d-electrons of the transition metals and on the atomic sizes [1]. Other structural properties such as equilibrium volumes, elastic constants and magnetic order were also addressed. Second, we investigate the electronic properties using the hybrid functional HSE06 to obtain a correct description of the gap, in order to identify which group of materials each compound belongs. As the top of the valence band is defined by the d-orbital, we can identify the class of the compound, i.e., insulator, conductor, etc., by the analysis of the band gap behavior in each crystalline phase. In conclusion, we performed an exploratory study of the TMDC properties that leads to an increase of the understanding of this class of materials, providing auxiliary information that can be used to identify candidate materials for new applications. [1] A. Kolobov and J. Tominaga, Two-Dimensional Transition-Metal Dichalcogenides (2016).
 
ID 50
Current-Voltage Modelling of Semiconductor Nanowire Resonant Tunneling Diodes
Rafael V. T. da Nobrega, Ulysses R. Duarte, R. Ragi, Murilo A. Romero
Departamento de Engenharia Elétrica e de Computação, Escola de Engenharia de São Carlos, Universidade de São Paulo
1D devices based on semiconductor heterostructures have attracted a great deal of interest in recent years. The technological evolution of device manufacturing techniques, i.e, top-down and bottom-up approaches [1-2], by means of a precise management of the composition layers and the doping concentration along the longitudinal dimension, have enabled the fabrication of 1D nanowires devices, including the manufacturing of Nanowire Resonant Tunneling Diodes (NWRTDs). However, if nanowires are to find widespread use in high-speed nanoelectronics applications, analytical device models, suitable for circuit-level simulations, are needed. The first model for the I-V characteristics of the RTD appeared in the seminal work of Tsu and Esaki in 1973 [3]. Later on, Schulman [4], using the formalism introduced by Tsu-Esaki, obtained an analytical expression for the I-V characteristics, unfortunately valid only for a specific RTD structure, considering a specific set of geometrical and material parameters. Their model required several empirical fitting parameters, with no direct physical meaning, to reproduce the experimental I-V curves [4]. From this starting point, we performed a systematic study of the phenomenological parameters presented by [4], to achieve a deeper understanding of their meaning, in such a way that the model could be applied to an arbitrary RTD structure. Next, we extended the model to describe the current transport to one-dimensional nanowire RTDs. In order to validate the proposed formalism, we compared our developed analytical current-voltage characteristics with experimental data available in the literature [1-2]. The results show good agreement regarding peak current, peak-to-valley ratio and diode behavior in the high-voltage regime of the device operation. [1]J. Wensorra. Nano Letters, 5, 2470, 2005. [2]M. Björk. Appl. Phys. Lett. 81, 4458, 2002. [3]R. Tsu. Appl. Phys. Lett. 22, 562, 1973. [4]J. Schulman. IEEE Electron Device Letters, 17, 220, 1996.
 
ID 51
GROWTH AND CHARACTERIZATION OF Zn1-xCoxO FILMS OBTAINED BY SPRAY PYROLYSIS
Yina Julieth Onofre Ramirez, Marcio Peron Franco de Godoy, Suelen de Castro, Ariano de Giovanni Rodrigues
Universidade Federal de São Carlos (UFSCar), Universidade de São Paulo (USP)
Co-doped ZnO have been recognized as promising materials in the field of dilute magnetic semiconductors. We present here a systematic study on the optical and structural properties of Zn1-xCoxO films obtained by spray pyrolysis with nominal Co concentrations from x = 0.03-0.15. The films were deposited on glass substrates at temperature range 220-300°C in atmosphere. The pulverized solution (0.01M) was prepared by dissolving appropriates amounts of dehydrated zinc acetate and tetrahydrated cobalt acetate in de-ionized water. The influence of cobalt doping on the structural and optical properties of Zn1-xCoxO films were studied by X-rays diffraction, optical absorption/transmission measurements and spectroscopy photoluminescence. The XRD patterns presented the peaks corresponding to wurtzite crystal structure for all polycrystalline films and indicated the absence of secondary phases of metallic Co and cobalt oxides. The films have a preferential orientation along the [002] direction and a crystallite size varying between 10-13nm, decreasing as doping concentration increases. The observed absorption peaks at 567, 612 and 654nm in the transmission and absorption spectra correspond to the d–d optical transitions, which reveals a substitution of Zn2+ ions in the ZnO lattice by Co atoms in +2 state. None absorption peak corresponding to cobalt oxides was observed. The PL spectra of the samples at the temperature range 12-300K presented three bands associated to near-band-edge absorption, structural defects and intratomic emissions of Co2+. Their optical emissions were characterized by the presence of traps that capture carriers and affect significantly the emissions. These traps have been already observed in undoped ZnO films at the range temperature 80-200K. In the Zn1-xCoxO films, the traps appear at lower temperature approx. 12–200K and through a qualitative model these were interpreted as potential fluctuations up to 17meV. Acknowledgments:CAPES, FAPESP and CNPq.
 
ID 52
Theoretical Photoluminescence spectra in III-V-TM ferromagnetic multilayers
Sara C. P. Rodrigues, Jefferson A. de O. Galindo, Guilherme M. Sipahi
Departamento de Física - Universidade Federal Rural de Pernambuco, Instituto de Física - Universidade de São Paulo
Diluted magnetic semiconductors (DMSs) are recently the most active area of research due to development of spintronic devices, which is related to their magnetic and transport properties. The DMSs are produced introducing magnetic elements in non-magnetic semiconductors; the principle is the injection of spin-polarized carriers. The materials commonly studied in the literature are II-VI and III- V group, which are doped with transition metals (Mn, Cr, V, and Fe). The quaternary alloy magnetic semiconductor $InAlGaAs:Mn$ has many potential advantages that cannot be realized by ternary alloy magnetic semiconductors. For instance, the band gap energy, easy magnetization axis, and band structure can be controlled by changing the In and Al content of $InAlGaAs:Mn$. A ferromagnetic ordering with a Curie temperature over 300 K of twenty layers of $InGaMnAs/InGaAs$ QW structure grown was found [1]. These findings open new possibilities to create new devices. In this work, we use the method $\vec{k}\cdot\vec{p}$ of Kane model $8 \times 8$. The solution is obtained self-consistently by the solution of the effective mass equation together with the Poisson equation. In this system we introduce the strain and exchange-correlation effects, which should be considered in the calculations. We investigate the behavior of system $AlGaAs/$(6 x $InAlGaAs/InAlGaAs: TM$). In particularly, we study the spin charge density and theoretical photoluminescence (PL) in these systems as a function of the variation of In and Al contents, as well as the acceptor donor concentration, $N_A$ and temperature. It is possible to observe a different behavior mainly for higher $N_A$ and as $\%$ of In increase. The main reason is due to the strain effects associated with the repulsive and magnetic potentials. Our results may provide a realist description for optical properties in these structures. $[1]$ Im Yoon $\textit{et al.}$, J. of Superconductivity and Novel Mag. $\textbf{28}$, 3049 (2015).
 
ID 53
Optical and spin properties of GaBiAs/GaAs QWs grown on (311)B GaAs substrates
G.A. Prando, V. Orsi Gordo, M.A.G. Balanta, H. V. A. Galeti, H. M. Alghamdi, M. Henini, J. Puustinen, M.Guina, F. Cadiz, A. Balocchi, H. Carrere, Y. Galvão Gobato
Federal University of São Carlos, Federal University of Uberlândia, School of Physics and Astronomy - Nottingham Nanotechnology and Nanoscience Centre - University of Nottingham, Optoelectronics Research Centre - Tampere University of Technology - Finland, INSA-CNRS-UPS - LPCNO- Université de Toulouse -France
III-V bismide heterostructures are interesting systems for possible applications in optoeletronics due to important increasing of spin-orbit (SO) band separation which could suppress the dominant non-radiative Auger recombination processes and consequently improve of photonic device efficiency in the near infrared region. This important increase in the SO interaction has also great potential to be exploited for semiconductor spintronics. Particularly, the spin properties of these Bi- compounds still remain largely unexplored, in spite of their possibility of engineering their spin-orbit coupling and subsequently manipulate the conduction electron spins . Furthermore, most of the previous works were focused on (100) Bi-related structures. The growth of high index substrates offers interesting possibilities as unusual crystal orientations are expected to allow, in a complementary way, for (i) an ultra-long, electrical controllable spin relaxation time and (ii) an efficient spin current transport. In this work, we have performed time resolved photoluminescence and magneto-photoluminescence measurements under high magnetic fields up to 15T in GaBiAs/GaAs QWs grown by molecular beam epitaxy (MBE) on (311)B GaAs substrates. Important effects of exciton localization were evidenced by PL and Magneto –PL measurements up to 15T. It was also found that the excitonic g- factor of (311) B GaAs1-xBix/GaAs QWs increases with the addiction of Bi as compared to GaAs usual factor. A bi-exponential PL decay was observed and associated to free and localized excitons. It was also observed that PL decay times decreases with the increasing of Bi% concentration, in addition, it was observed an important enhancement of circularly polarization degree with the decreasing of laser intensity (condition of emission dominated by localized excitons). This result was mainly associated by important increase of PL decay times due to exciton localization by disorder.
 
ID 54
The role of short-range magnetic correlations in the gap opening of topological Kondo insulators
Marcos S. Figueira, Edwin Ramos, Roberto Franco, Jereson Silva-Valencia, Mario E. Foglio
Universidade Federal Fluminense, Universidade Nacional da Colombia, Universidade Estadual de Campinas
In this article we investigate the effects of short-range anti-ferromagnetic correlations on the gap opening of topological Kondo insulators. We add a Heisenberg term to the periodic Anderson model at the limit of strong correlations in order to allow a small degree of hopping of the localized electrons between neighboring sites of the lattice.  This new model is adequate for studying   topological Kondo insulators, whose paradigmatic material is the compound $SmB_{6}$. The main finding of the article is that the short-range antiferromagnetic correlations, present in some Kondo insulators, contribute decisively to the opening of the Kondo gap in their density of states. These correlations are produced by the interaction between moments on the neighboring  sites of the lattice. For simplicity, we solve the problem on a two dimensional square lattice. The starting point of the model is the $4f-Ce $ ions orbitals, with $J=5/2$ multiplet in the presence of spin-orbit coupling. We present results for the Kondo and for the antiferromagnetic correlation functions. We  calculate the phase diagram of the model, and as we vary the $E_{f}$ level position from the empty regime to the Kondo regime, the system develops  metallic and topological Kondo insulator phases. The band structure calculated shows that the model describes a strong topological insulator.
 
ID 55
Analytical calculation of the Green’s functions for the monolayers materials nanoribbons
Renan Bento Ribeiro Campos, Marcos Sergio Figueira
UFF
After the experimental realization of graphene [1], which is a monolayer honeycomb structure of carbon atoms, the search for other monolayer materials became one of the most active fields in condensed matter physics. Other examples of promising monolayer materials are silicene, rmanene and stanene, which are monolayer honeycomb structures of silicon, germanium and tin, spectively [2]. Those last materials are expected to exhibit topological insulator behavior, which is characterized by an insulating gap in the bulk accompanied by topologically protected gapless edge states [3]. Our first step in the direction of the study of these systems, is the analytical derivation of graphene Green's functions [FG] nanoribbons for both types of structures: ``armchair" and ``zig-zag". We present results for the density of states of ``armchair" nanoribbons with any value of width N and for $N=2$ ``zig-zag" nanoribbon, which forms a honeycomb chain. This last structure is closely related to the oligoacenes which are organic molecules that consist of fused benzene rings chains [4]. As the number of fused molecular units increases, the optical gap either approaches a constant value or vanishes. In the former case, the infinite chain is a band insulator and in the latter it is a metal. In the case presented here, the $N=2$ infinite ``zig-zag" nanoribbon presents a metallic behavior with a double peak at the Fermi energy. [1] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov, Science  306, 666 (2004). [2] Motohiko Ezawa, Journal of the Physical Society of Japan  84, 121003 (2015). [3] M. Z. Hasan and C. Kane, Rev. Mod. Phys. 2, 3045 (2010). [4] Richard Koryta\'{r}, Dimitra Xenioti, Peter Schmitteckert, barek Alouani and  Ferdinand Evers, Nature Communications DOI: 10.1038/ncomms6000.
 
ID 56
Edge Phonons in MoS$_{2}$ Nanoribbons: Ab initio calculations
Igor Daniel Evangelista, Pedro Venezuela
Universidade Federal Fluminense
Two-dimensional (2D) materials, such as well-known graphene, are increasing their importance due to their unusual physical properties and potential technological applications. In the particular case of electronic and opto-electronic applications the development of semiconducting 2D materials is highly desireable. Molybdenum disulfide (MoS22) is a direct band gap semiconductor of the type MX22 called Transition Metal Dichalcogenide (TMD), where M is a transition metal atom (Mo, W, Cr, etc) and X is a chalcogen atom (S, Se and Te). Due to its peculiar properties, MoS22 may have important applications in electronics, optoelectronics and photonics. The vibrational properties are also important to be studied in these materials, and in this work we were interested to investigate the edge phonons in MoS22 nanoribbons. Density Functional Theory (DFT) was used via the Quantum Espresso package to study the electronic and structural properties as well as the behavior of phonons in MoS22 nanoribbons with either zigzag or armchair terminated edges. In armchair and zigzag edges the phonons have different frequencies and symmetries. This property may be used to characterize MoS22 nanoribbons using vibrational spectrocospy such as Raman and Infrared Absorption. The vibrational properties of MoS22 nanoribbons can also be important to access the structural stability of these materials.
 
ID 57
Interplay of structure asymmetry, defect induced localization and spin-orbit interaction in Mn doped quantum dots
L. Cabral, Fernando P. Sabino, Vivaldo Lopes-Oliveira, Juarez L. F. Da Silva, Matheus P. Lima, G. E. Marques, Victor Lopez-Richard
Departamento de Física - Universidade Federal de São Carlos, Instituto de Física de São Carlos - Universidade de São Paulo, Instituto de Química de São Carlos - Universidade de São Paulo
Intertwining asymmetries and external magnetic fields in semiconductor nanostructures is a complex theoretical endeavor. In this context, the study of doped semiconductor quantum dots with magnetic impurities is a challenging task when adding spin-orbit interaction and exchange interaction with asymmetry effects. Thus, the main objective of the present work is to combine all these factors within a single theoretical framework, allowing the introduction of external fields. In particular, the asymmetry tuning of the effective Zeeman splitting and the ground state character are characterized. Contrasting different symmetry configurations allows building a realistic picture of the plausible ways the Mn is incorporated in the system. The effective mass model of the electronic structure that allows combining confinement profiles with controllable symmetry lowering, spin-orbit interaction effects and external fields under a variety of configurations in a systematic way has been complemented with atomistic simulations. The connection between the effective mass model and such atomistic approaches was done through a characterization of the main trends for the Mn positioning, as well as the exchange interaction term, which is calculated with a fully ab-initio technique. This approach allowed assessing how the hybridization process that takes place at atomistic levels is affected by strain and confinement environment.
 
ID 58
Structural, Electrical and Optical Characterization of Indium Doped TiO2 Thin Film deposited on Si Substrate
Jorlandio F. Felix, Noor alhuda Al Saqri, Aniruddha Mondal, Yara Galvão Gobato, Vanessa Orsi Gordo, Faisal Al Mashary, Mohamed Henini 
Institute of Physics-Universidade de Brasília, Department of Physics - Sultan Qaboos University - Oman, National Institute of Technology Agartala -Department of Electronics and Communication Engineering - India, Departamento de Física - Universidade Federal de São Carlos, School of Physics and Astronomy - Nottingham Nanotechnology and Nanoscience Center - University of Nottingham
Transition metal (TM) oxides such as $TiO_2$ constitutes an important class of inorganic solids exhibiting a very wide variety of structures, electronic and optoelectronic properties [1]. It has great importance for several technological applications such as heterogeneous catalysis, corrosion-protective coating of metals, microelectronic devices, anti-fogging application on car glass and photo-electrochemical (PEC) water splitting applications [2].  In this work, we have investigated structural, electrical and optical properties of  In doped $TiO_2$ films. The thin films (TF) samples were grown by e-beam on Si substrates and the In doping was obtained by thermal evaporation of In on $TiO_2$. The electrical contacts were processed in form of circular mesas with diameter of 700 $\mu$ m. The structural properties of the samples were investigated by using X-Ray powder diffraction and Raman Spectroscopy. It was observed that as we increase the In doping the crystal structure of TiO2  phase changes from anatase to rutile phase. The introduction of defect levels by In doping was investigated by deep level transition spectroscopy (DLTS) and photoluminescence (PL) techniques.  It was observed that the doping concentration was increased from $3.62×10^{14}$ $cm^{-3}$ to $1.79×10^{15}$ $cm^{-3}$ for 5 nm In/$TiO_2$ TF to 50 nm In/$TiO_2$ TF devices, respectively. A unique behaviour of reduction in trap concentration was evidenced for the sample with higher In doping concentration. The PL spectrum shows a broad band in visible region which was associated to defect emissions. A red shift of PL peak position with the increase of In concentration was observed. Particularly, for the  5 nm In/$TiO_2$ TF sample the  emission peak is around 2.4 eV while for  the 50 nm In/$TiO_2$ TF is around  1.9 eV.  [1] M. Ni, M.K.H. Leung, D.Y.C. Leung, K. Sumathy, Renew. Sust. Energ. Rev. vol. 11, 401-425, 2007. [2]Y. Xie, at al., Nanotechnology vol. 25,  075202, 2014.
 
ID 59
Edge impurities in the superconducting Kane-Mele model
Raphael Levy Ruscio Castro Teixeira, Dushko Kuzmanovski, Annica Black-Schaffer, Luis Gregório Godoy de Vasconcellos Dias da Silva
Instituto de Física - Universidade de São Paulo, Uppsala University
We studied a finite sample with honeycomb lattice using the Kane-Mele. We studied a finite sample with honeycomb lattice using the Kane-Mele model. We added superconductivity and impurity in the Hamiltonian and analyzed how it affected the energies levels, the superconducting order parameter and free-energy. We also show that for a chain with 10 impurities there are zero-energy mode at the end of the chain, that might be Majorana fermion.
 
ID 60
Optimization of the InGaP Top ${\it pn}$ Junction for a New Triple Tandem Solar Cell Design
Victor de Rezende Cunha, Patrícia Lustoza de Souza, Rudy Massami Sakamoto Kawabata, Luciana Dornelas Pinto, Mauricio Pamplona Pires
Pontifícia Universidade Católica do Rio de Janeiro, Universidade Federal do Rio de Janeiro
The solar cells with highest efficiencies are the tandem cells with three or four ${\it pn}$ junctions in series, connected by tunnel diodes. The current produced by the stacked ${\it pn}$ junctions is limited by the smallest one. Therefore, to optimize the full solar cell efficiency it is crucial to match the current produced by the different junctions. This issue has been extensively investigated over the years. In the most usual case of three ${\it pn}$ junctions, the current is limited by the middle junction. Recently, the use of multiple quantum wells to further improve the current generated at the intermediate ${\it pn}$ junction was proposed. When such a middle junction is used, new optimization of the top cell is required to reach a better current matching. In this work we have used a commercial software, Comsol, to optimize the top solar cell for a triple junction structure to be used in space applications, meaning subjected to the AM0 spectrum. The newly designed solar cell is based on an InGaP ${\it pn}$ junction with AlGaInP ${\it n}$-doped window. The layers are lattice matched to GaAs and to Ge, which is the material used for the bottom ${\it pn}$ junction. These materials were chosen based on the fact that they are more resistant to radiation, which is of paramount importance for use in satellites. The first step to fabricate the designed solar cell is the optimization of each individual layer.  The different InGaP and AlGaInP semiconductor layers of the designed solar cell have been grown by metalorganic vapor phase epitaxy at 675 ºC. The alloys' composition was calibrated. High doping levels of InGaP were achieved. However, difficulties in reaching a doping level of the AlGaInP window layer of around $1 x 10^{18}cm^{-3}$, as required, were faced and should be discussed.  Additionally, luminescence and x-ray diffraction data of the grown samples will be presented.
 
ID 61
Discerning Intraband Absorption Spectra Techniques for QDIPs and QWIPs
Eric Hermanny, Patrícia Lustoza de Souza, Rudy Massami Sakamoto Kawabata, Mauricio Pamplona Pires
Pontifícia Universidade Católica do Rio de Janeiro
There is vast bibliography on FTIR spectroscopy focused on its use in chemistry, like detecting the presence and concentration of substances in samples such as solutions or crystals, but very little when it comes to the characterization of infrared optoelectronic semiconductor devices. This is probably why there is a somewhat generalized belief that one can satisfactorily use a well-known single-beam FTIR method for obtaining absorption spectra of QWIPs also for characterizing QDIPs. Apparently, it is a very convenient method because there is no need to manipulate the sample between s and p scans, only the polarizer must be adjusted. But the difference in the selection rules that characterize the intraband absorption of radiation by QWs on one side, and by QDs on the other, advises the use of a special, more complex method for obtaining absorption spectra of QDIPs from the FTIR data. We present a special single-beam FTIR method for obtaining intraband absorption spectra of detector structures based on QDs. The main difference in the proposed technique is that, instead of using the s-polarized absorption spectrum as a background for the p-polarized spectrum, it uses the scan of the substrate alone, without the active region, as background for the scan of the sample. The theory behind this method is described and the spectral processing needed to overcome possible laboratorial hindrances that the technique implies is suggested. As an extension of this development, we show how this technique can produce spectra that reveal whether on the electronic transition involved in each intraband absorption peak there is a change in angular momentum. Finally, a case study is presented.
 
ID 62
Optical properties of Te and Be doped self-catalyzed GaAs nanowires grown on Si
Marcelo Rizzo Piton, Eero Koivusalo, Teemu Hakkarainen, Soile Suomalainen, Sergio Souto, Helder V. A. Galeti, Ariano D. Rodrigues, Yara G. Gobato, Mircea Guina
Federal University of São Carlos, Tampere University of Technology, University of São Paulo
Semiconductor nanowires (NWs) are interesting systems for nanoscale device applications in the area of electronics, photonics, mechanics, and sensors. However, for practical applications it is necessary an effective control of doping process of these systems as it results in changes of their electrical, optical, and magnetic properties. In this work, we have investigated structural, optical and electrical properties of Te and Be doped GaAs NWs grown by Au-free self-catalyzed molecular beam epitaxy route in order to obtain n- and p-type nanostructures. The NWs were grown directly on Si(111) substrates using a novel technique based on lithography-free Si/SiOx patterns fabricated by a self-assembled method, which allows synthesis of highly uniform NWs with controllable size and diameter. The incorporation of Te- and Be-dopants in GaAs NWs was investigated by High resolution transmission electron microscopy (HTEM), transport, photoluminescence (PL) and Raman spectroscopy techniques. It was observed that the PL spectrum is very complex presenting several peaks attributed to exciton, band-acceptor transition due to residual carbon, impurity related transition, etc. Furthermore, both ZB and WZ phases are formed in the growth of doped NW contributing to the PL peaks. Our PL results have shown a blue shift and incorporation of additional bands in the PL spectrum as we increase the Te and Be doping level. These results indicate that the carrier concentration increase in the NWs yields an enhancement in the complexity of the doping and on the WZ and ZB phase mechanism. Raman spectroscopy was carried out to analyze the effect of the dopant incorporation in GaAs NWs. Our results have shown an important reduction in LO Raman peak intensity with the increasing of the dopant concentration. This result was mainly associated to an increase of free carrier concentration which reduces the surface depletion layer in the NWs.
 
ID 63
Field Effect Transistors based in a Sn3O4 nanobelt
Rosana Alves Gonçalves, Leonardo Martins do Amaral, Maurício Ribeiro Baldan, Olivia Maria Berengue, Adenilson José Chiquito
Universidade Estadual Paulista (UNESP), Universidade Federal de São Carlos (UFSCAR), Instituto Nacional de Pesquisas Espaciais (INPE)
The $Sn_3O_4$ is a non-stoichiometric oxide composed by two different states of oxidation (SnO e $SnO_2$ structured in layers) with band gap in the visible portion of electromagnetic spectra. For this reason, this material is a potential candidate for applications in opto-electronic devices and gas sensors and also in field effect transistors. In this work $Sn_3O_4$ nanowires were grown in a horizontal tubular furnace through of carbothermal reduction method using $SnO_2$ powder as precursor. Morphological and structural characterization of samples were performed by Scanning Electronic Microscopy (SEM) and X-Ray Diffraction (XRD). Semiconductor character of nanobelts was confirmed by temperature dependent resistivity measurements. The variable range hopping (VRH) was detected as main mechanism of electronic transport in these samples in a large range of temperatures. Taking into account the electronic transport measurements we built a Metal-oxide Semiconductor Field Effect Transistor (MOSFET) based in an unique belt of $Sn_3O_4$. The transistor was characterized by $I_{DS}\quad versus\quad V_{DS}$ with fixed gate and $I_{DS}\quad \mathrm{x}\quad V_{G}$ where it was seen that the channel ($Sn_3O_4$) is n-type with a carriers density of $5,07\quad \mathrm{x}\quad 10^{17}\quad cm^{-3}$. The field effect properties of the $Sn_3O_4$ nanowires were also studied under UV illumination. The results indicate that the conductance is an order of magnitude greater in this condition than in dark. Photoconductive behaviour in these samples was associated to the presence of oxygen vacancies in $Sn_3O_4$ structure. From this work we can suggest that the $Sn_3O_4$ nanowires are candidates to be active channels in field effect transistors and UV light sensors.
 
ID 64
The role of defects on the efficiency of intermediate band solar cells based on III-V semiconductors
Lida Janeth Collazos Paz, Maryam Al Huwayz, Faisal Al Mashary, Luciana Dornelas Pinto, Mohamed Henini, Patricia Lustoza de Souza, Maurício Pamplona Pires
LabSem - Pontifícia Universidade Católica do Rio de Janeiro, School of Physics and Astronomy - University of Nottingham
Solar cells with the highest efficiency are the tandem cells which consist of four p-n junction in series, connected by tunnel diodes. Such cells have efficiencies up to 46% [1] but are expensive and for this reason they are used only in large plants or in space applications. One alternative to substitute the tandem cells and reach even higher efficiencies is the intermediate band solar cell (IB-solar cell) which consists of one p-i-n junction with an intermediate band produced by the coupling of quantum dots which are incorporated into the i-region. According to theoretical calculations their efficiency could reach 63% [2]. Although such a high efficiency has been predicted, it has not been experimentally achieved, most likely due to the presence of undesired recombination centers and/or carrier traps. In this work we have investigated the individual layers comprising an IB-solar cell based on III-V semiconductors in order to identify the recombination centers and/or electrically active carrier traps lying within the band gap which could affect the solar cell performance. Different trap levels have been detected and identified in the different layers. In addition to the recombination centers, the obtained results show evidence that the required growth temperature cycle for the nucleation and capping of the quantum dots introduce additional defects which are deleterious for the solar cell perfomance. These results should help in choosing more appropiate quantum dots growth conditions in order to minimize the incorporation of such carrier traps to reach high efficiency. References [1] Tibbits, Thomas ND, et al. "New efficiency frontiers with wafer-bonded multi-junction solar cells." 29th European PV Solar Energy Conference and Exhibition. Amsterdam, Netherlands, 2014. [2] Antonio Luque and Antonio Martí. Increasing the efficiency of ideal solar cells by photon induced transitions at intermediate levels. Physical Review Letters, 78(26):5014, 1997.
 
ID 65
Asymmetric superlattice quantum well infrared photodetector with dual-mode operation
Pedro Henrique Pereira, Germano Maioli Penello, Mauricio Pamplona Pires, Deborah Sivco, Claire Gmachl, Patricia Lustoza de Souza
Pontificia Universidade do Rio de Janeiro, Universidade do Estado do Rio de Janeiro, Universidade Federal do Rio de Janeiro, Princeton University
A particular high-performance InGaAs/InAlAs asymmetric superlattice quantum well infrared photodetector with photovoltaic (PV) and photoconductor (PC) dual-mode operation has been fabricated and characterized in the work reported here. The heterostructure was designed to absorb mid-infrared radiation by self-consistently solving the Schrödinger-Poisson equations for the conduction band. The active region consists of 7 InGaAs quantum wells separated by a 7.0 nm thick InAlAs barrier. The quantum wells are undoped and with width of 2.0 nm, except for the sixth one which is $\textit{n}$-doped to $3.0x10^{18} cm^{-3}$ and has thickness of 2.5 nm. This quantum well behaves as a defect on the otherwise periodic structure giving rise to localized states in the continuum. The sample was grown by molecular beam epitaxy. Photocurrent spectra were measured by Fourier Transform Infrared Spectroscopy (FTIR) at 77 K. In the photovoltaic mode, meaning zero applied bias, the photocurrent spectrum presents two narrow peaks at 300 meV and 440 meV. The PV mode was achieved due to the structural asymmetry which creates localized states in the continuum with preferential direction for electrons flow. For the photoconductive mode, the photocurrent spectrum presents the linewidth strongly dependent on the direction of the applied bias. The photocurrent spectrum with negative biases shows two clear peaks at 260 meV and 300 meV. In this case, the electrons flow to the substrate. For positive bias, the photocurrent spectrum presents one main peak with energy at 300 meV and a lower energy shoulder. The experimental results are in excellent agreement with the calculated absorption spectra.
 
ID 66
Reciprocal space maps of CdTe and CdMnTe thin films grown on Si(111)
Sukarno Olavo Ferreira, Gilberto Rodrigues da Silva Júnior
Universidade Federal de Viçosa
CdTe is a direct gap semiconductor with important eletro-optical properties. In this work, CdTe and CdMnTe epitaxial layers were grown by molecular beam epitaxy on Silicon (111) substrates. The influence of parameters like substrate temperature and Mn concentration were investigated by high resolution x-ray diffraction. Reciprocal space maps near the (111) and (224) reflections were measured using a D8 Discover diffratometer equipped with a Ge(220) monocromator and a 192 element linear detector. The results show that the CdTe layers show a tilt angle in relation to the substrate between 0.1and 0.3 degrees. Sample quality is improved as substrate temperature increase and also when Mn concentration increases up to about 30%. Introduction of a higher Mn concentration starts to decrease the crystalline quality. This work has been supported by FAPEMIG, CNPq and CAPES.
 
ID 67
Light-induced coherent spib precession in singly-charged quantum dots
A. R. Naupa, C. F. H. Corstjens, P. M. Koenraad, F. W. M. Otten, A. B. Henriques
Universidade de Sao Paulo, Eindhoven University of Technology
In the semiconductor research community, one goal is to understand the magnetization dynamics of the carrier spin in quantum dots (QDs), to utilize these spins in spin-based technologies. In the case of negatively doped QD ensembles the spin of the resident electrons are incoherent, leading to an overall zero magnetization. However, by illuminating these QDs with ultrashort light pulses, it is possible to generate a net magnetization. The process of light-induced magnetization was first explained using a simple semiclassical model [1]. However, this model contradicts some experimental results. Recently, a more complete quantum mechanical model was developed [2], which describes well both the phase and amplitude of the photoinduced magnetization precession as a function of the applied magnetic field. Remarkably, the trion lifetime ττ is the one and only adjustable parameter that fits simultaneously both dependencies. The trion lifetime is usually measured by time resolved photoluminescence of a single dot, requiring another method to measure ττ in samples with high QD density. The purpose of this work is to establish whether a fit of the amplitude and phase of the precessing coherent magnetization on magnetic field, using theory in [2], is a practical method to yield a reliable measure of ττ. Using time-resolved Faraday rotation (TRFR), the phase and amplitude of the photoinduced magnetization was investigated in a large number of QD samples containing singly negatively charged QDs. In all cases, the theory fitted the data well. However, we find that the dispersion in the trion recombination times extracted by the TRFR technique is very high, which implies that the uncertainty in the value of ττ measured can be large. [1] S. Varwig, A. Greilich, D. R. Yakovlev, and M. Bayer. Phys. Stat. Sol. B 2251, 1892 (2014), [2] A. B. Henriques, R. C. Cordeiro, P. M. Koenraad, F. W. M. Otten, and M. Bayer, Phys. Rev. B 91, 081303 (2015).
 
ID 68
The structural properties of the protective layer of microlamps under polarization
V. R. S. Cassimiro, R.M. Cunha Júnior, G.P. Rehder, I. Pereyra, M.N.P. Carreño, M. I. Alayo, M.C.A. Fantini, N. Trcera
Laboratório de Cristalografia - Instituto de Física - Universidade de São Paulo, Laboratório de Microeletrônica - Escola Politécnica - Universidade de São Paulo, Société Civile Synchrotron SOLEIL - Lucia Beamline
Microlamps have many applications, such as: sources for integrated optics, infrared micro spectroscopy and localized thermal treatment sources. The main objective of this work was to build microlamps and to analyze the structural modifications induced on the protective layers of the metallic filament by the heating process. Their fabrication consists on films deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) and sputtering over silicon substrates. The filament, composed by a thin chromium wire, is protected against oxidation by a top thin layer. Four different materials were used as protective layer: a-SiC:H, SiOxNy, AlN and TiO2. The protective film is heated by the metallic filament and their chemical and structural properties may change, depending on the time interval and intensity of the applied current (up to 2h and 50mA). X-ray absorption near edge spectroscopy (XANES) measurements allowed investigating changes on the properties of the microlamps protective films heated under different polarization conditions. The LUCIA beam line of the synchrotron SOLEIL has a microfocus spot (3x3µm), permitting to evaluate the small thermally affected zone. The results showed that SiOxNy film is thermally stable with negligible changes on the XANES spectra. A slight AlN oxidation is observed as heating rises which is evident for the sample heated at extreme conditions. TiO2 XANES spectra showed that the material is crystallized on rutile structure and is also thermally stable. SiC thin films were widely affected showing an oxidation process as the time interval and intensity of the current increase. In addition, once the films were deposited over the Cr filament, their XANES spectra are quite different from the standard sample (deposited over Si), even for the non-polarized microlamp, indicating a Cr contamination on the SiC structure. For technical purposes, the SiOxNy and TiO2 films are suitable as filament protective layer due to their high thermal stability.
 
ID 69
Transport properties of van der Waals hybrid structures
M. Pacheco, P. A.Orellana, A. B. Felix, A. Latgé
Universidad Técnica Federico Santa María, Universidade Federal Fluminense
Here we study transport properties of van der Waals heterostructures composed of carbon nanotubes adsorbed on nanoribbons of distinct 2D materials. Calculations of the electronic density of states and conductance of the hybrid systems are obtained in single band tight-binding approximation in the Green function formalism by adopting real space renormalization schemes. We show that an analytical approach may be derived when both systems are formed by the same type of atoms. In the coupled structures the different electronic paths along the ribbons and finite nanotubes lead to quantum interference effects which are reflected as Fano antiresonances in the conductance. The electronic and transport properties of these materials are modulated by changing geometrical and structural parameters, such as the nanotube diameter and the widths and edge type of the ribbons.
 
ID 70
A route to efficiently implement quantum gates in semiconductor nanostructures
Leonardo Kleber Castelano, Emanuel Fernandes de Lima, Justino Ruas Madureira, Marcos Henrique Degani, Marcelos Zoega Maialle
Departamento de Física - Universidade Federal de Sao Carlos, Faculdade de Ciências Integradas do Pontal Universidade Federal de Uberlândia - Ituiutaba, Universidade Estadual de Campinas - FCA- Limeira.
Semiconductor nanostructure is a platform that have been used to implement quantum information protocols due to its possible scalability. In particular, quantum dots have shown the possibility of storing quantum information in both degrees of freedom charge and spin. To demonstrate the capability of a system to be used as a platform for quantum computation, one has to prove that it is possible to create the universal set of quantum gates (X, Z, H, and CNOT). The implementation of a quantum gate employs pulsed external fields that drive the dynamics of the system. Usually, these pulsed fields have simple shapes, e.g. squared or sinusoidal pulses. The optimal quantum control theory has the purpose of finding a special field that maximize a certain observable. In this study, we employ the optimal quantum control theory to find pulsed fields that efficiently implement the universal set of quantum gates in double quantum dots with two electrons. Our results demonstrate that these tailored pulses are able to implement the universal set of quantum gates with high fidelity, even though incoherent processes are taken into account.
 
ID 71
Tailoring Quantum Wells for the Implementation of Two-Subband Persistent Spin Helices
Amina Solano Lopes Ribeiro, Flavio C. D. Moraes, Gennady M. Gusev, A. K. Bakarov, Gerson J. Ferreira, Felix G. G. Hernandez 
Universidade de São Paulo, Universidade Estadual de Novosibirsk, Universidade Federal de Uberlândia
Spin-electronics (spintronics) takes advantage of the quantum spin degree of freedom to create integrated devices with new functionalities. Moreover, studies of electron transport by drift or diffusion in semiconductor nanostructures are still required for the implementation of practical technologies. For two-dimensional electron gases (2DEGs) hosted in a semiconductor quantum well (QW), several reports explored the spin-orbit interaction tunability to produce a unidirectional spin-orbit field for the diffusive generation of a spin helix. Very recently, the drift in those helical systems was also demonstrated showing remarkable properties. For two-subband systems, the inter- and intra-subband spin-orbit constants (SOCs) were extensively studied, including a proposal for a crossed persistent spin helix (cPSH) and an intrinsic mechanism for edge spin accumulation. Experimentally, we studied here a 2DEG confined in a symmetrically doped wide QW producing a two-subband electron system with strong intersubband scattering. The sample was characterized using magneto-transport and optical techniques. From the first, the electron mobility and total sheet and subband densities were obtained. All the relevant SOCs were calculated using a random walk model incorporating Rashba, linear and cubic Dresselhaus, and intersubband spin-orbit couplings. Moreover, the possibility of a two-subband spin helix was explored. For the later technique, time- and space-resolved Kerr rotation was performed using a pump-probe scheme. We determined the electron g-factor and spin relaxation time dependence on the experimental parameters and related both with the relaxation mechanisms.
 
ID 72
Spin mobility in a 2DEG with Persistent Spin Helices
Felix G. G. Hernandez, M. Luengo-Kovac, F. C. D. Moraes, G. J. Ferreira, A. S. L. Ribeiro, G. M. Gusev, A. K. Bakarov, V. Sih
Universidade de Sao Paulo, University of Michigan, Universidade Federal de Uberlândia, Novosibirsk State University
The pursuit of a new active semiconductor component based on flow of spin, rather than that of charge, strongly motivates research in the spintronics field. Recently, studies have been developed to make a spin transistor robust against spin-independent scattering including the Rashba and Dresselhaus spin-orbit interactions. For example, it has been demonstrated that SU(2) symmetry can be obtained when the strengths $\alpha$ and $\beta$ are equal. In this case, the resulting spin-density texture is a persistent spin helix (PSH). A two-dimensional electron gas hosted in a semiconductor quantum well with two-subbands occupied introduces new characteristics to the PSH dynamics and offers unexplored opportunities, for example, to create a set of crossed PSHs. Here [1], we experimentally study spin drag in a system where the spin-orbit couplings (SOCs) were tailored in order to attain ortogonal PSHs almost simultaneously: the first subband in the PSH+ and the second subband in the PSH-. The sample consists of a single wide GaAs QW with high charge mobility and total density of 7x10$^{11}$ cm$^{-2}$. A device was fabricated in a cross-shaped configuration with channels along the [1-10] and [110] directions and with lateral and top gate contacts. We measured the spin polarization using time resolved Kerr rotation as function of the space and time separation of pump and probe beams. With the application of a drifting in-plane electric field, we observed highly anisotropic spin-orbit fields in the range of several mT and spin mobilities of approximately of 2x10$^5$ cm$^2$/Vs. We were able to control the SOCs in both subbands as function of gate voltage and to determine an inverse relation for the spin mobility dependence on the SOCs. We directly revealed the resistance experienced in the transport of a spin package in anisotropic fields. We modeled our results using a random walk approach and obtained excellent agreement. [1] M. Luengo-Kovac et al. arXiv:1703.08405 (2017).
 
ID 73
Confinement and Fermion doubling in topological insulators
Bruno Messias Farias de Resende, Edson Vernek, Gerson J. Ferreira
Federal University of Uberlândia
Linear in momentum Hamiltonians suffer from the fermion doubling problem when put on a lattice (i.e. finite differences approximation). This issue leads to low energy spurious states, called "doublers", that affect all physical phenomena set near the Fermi level, yielding wrong results. For instance, it gives spurious degeneracies that doubles the value of the zero-temperature conductance with respect to the correct continuous model. Furthermore, it may lead to false interferences. Another non-trivial issue with linear Hamiltonians is the confinement. Due to Klein tunneling, simple electrostatic potentials do not lead to confinement barriers. Instead, the confinement potential needs to open a gap on the outer region of the system (e.g., circular dots, ribbons). However, a gap opening is associated with a broken symmetry (e.g. time reversal). Consequently, for linear Hamiltonians the confinement also breaks some symmetry. In this work we proposed a method based on Wilson's mass term to overcome the fermion doubling problem, such that it also leads to a hard-wall confinement associated with the desired broken symmetry. We apply the model to investigate the conductance over ribbons set on thin films of a 3D topological insulator (Bi2Se3), considering the leads to be metal/Bi2Se3 junctions, while the scattering region is pure Bi2Se3. We show that reflections at the leads yield narrow Fabry-Pérot resonances, whose correct double degeneracy is assured by the choice of Wilson's mass. This degeneracy may lead to the Kondo effect seen in a recent experiment. The authors acknowledge support from FAPEMIG, CAPES and CNPq.
 
ID 74
Fabrication of interdigital transducers and growth of ZnO films for the generation of surface acoustic waves
Odilon D. D. Couto Jr., João Vitor Martins Fernandez, Rodrigo Oliveira Tambellini, José Maria C. da Silva Filho, Francisco das Chagas Marques, Luana C. J. Espindola, Aline Pascon
Instituto de Física - Universidade Estadual de Campinas, Centro de Componentes Semicondutores - Universidade Estadual de Campinas
Surface acoustic waves (SAWs) have emerged in the past few years as a powerful and non-destructive tool to achieve modulation of the optical properties and to change carrier and spin dynamics in semiconductor light emitting nanostructures. The electron-hole interaction in optically generated carriers is strongly affected by the propagating SAW piezoelectric potential and strain field. Conventional III-V semiconductors like GaAs, however, present weak piezoelectricity. In this way, it is often desirable to couple III-V semiconductor nanostructures with highly piezoelectric materials, like ZnO, in order to achieve efficient carrier/spin manipulation. In this contribution, we design and fabricate aluminum interdigital transducers (IDTs) for the generation of SAWs on ZnO thin films and LiNbO3 substrates. IDTs are designed to operate between 100 and 500MHz. Fabrication is performed using optical photolithography followed by aluminum evaporation and lift-off. ZnO thin films are grown by atomic layer deposition (ALD) on Si substrates. We investigate the effects of the growth conditions on the crystal orientation of the films, which is directly related to its piezoelectricity. We show that growth temperatures around 200oC yield the highest fraction of c-oriented ZnO cristallites. We also simulate the acoustic properties of IDTs on ZnO films and LiNbO3 substrates using finite element method (FEM). We demonstrate how to calculate the acoustic resonances and the piezoelectric properties for our structures. In particular, we show that ZnO films with thicknesses of 500 to 600 nm on Si should present an enhancement in the amplitude of the acoustic fields due to mode guiding on the ZnO layer, which is very interesting for electron transport in nanostructures placed on sample surface.
 
ID 75
Optical transitions of wurtzite phase GaP asymmetric nanowires
Fernando Iikawa, Bruno C. da Silva, O. D. D. Couto, Jr., L. F. Zagonel, M. A. Cotta, L. C. O. Dacal, M. M. de Lima Jr., A. Cantarero
IFGW-Unicamp, IEAv-CTA, University of Valencia
 Indirect gap cubic GaP becomes direct gap in wurtzite (WZ) phase nanowire structures. WZ-GaP is thus a promising material for nanostructured optoelectronic devices. Recently, a direct band gap (or pseudogap) energy of 2.17 eV has been estimated from photoluminescence (PL) measurements on WZ-GaP nanowires [1]. High-energy transitions have also been observed by resonant Raman scattering [2]. However, the accurate experimental values of the splitting energies of both the conduction (CB) and valence band (VB) are still unknown. In this work, we present the energies of several optical transitions of WZ-GaP asymmetric nanowires obtained by using photoluminescence excitation (PLE) technique. The nanowires are grown on GaAs substrate by vapor-liquid-solid in a chemical beam epitaxy system using gold as catalyst. Instead of the typical nanowire shape, our nanowires present a knife-like morphology and are predominantly in the WZ-phase with few zinc-blende (ZB) regions. The PLE spectra present strong absorption edges on approximately 2.37 and 2.84 eV and a very weak one in the region between 2.1 and 2.2 eV. Due to the week and noisy signals, the pseudogap energy is not accurately measured, ~2.18 eV, and A-B splitting is no longer resolved. We tentatively attribute the absorption edge at ~2.37 eV to the $\Gamma_8c$–C-band transition and the absorption edge at ~2.84eV to the $\Gamma_7c$–A-band. The latter is allowed by dipole transition. This result agrees quantitatively with the theoretical energies reported in previous works [1], however, it is in disagreement with the analysis of the results of resonant Raman scattering [2]. Therefore, further calculations of absorption spectra are needed to identify precisely the origin of each transition shown in PLE spectra. References [1] Assali, et al, J. Applied Phys. 120, 044304 (2016), [2] J. K. Panda, et al, Appl. Phys. Lett. 103, 023108 (2013)
 
ID 76
Planar double quantum dot with two-electron occupation
Marcos H. Degani, Marcelo Z. Maialle
Faculdade de Ciências Aplicadas - UNICAMP
Double quantum dots (DQDs) in semiconductor structures are interesting physical systems to investigate the dynamics of the quantum states. In particular, when the DQD energy levels are controlled by external gate voltages, the interaction between the pair of dots can be modified to created energy level alignment resulting in resonant tunneling current through the DQD. The particle occupations of each dot affects the tunneling current through the Coulomb blockade effect, and the spin components of the particles in the dots also affect the current, as for instance via the Pauli blockade. These characteristics have been explored to use DQDs as platform for quantum computation, in which either charge or spin can play the role of the quantum bit (qubit). In this work we solve a model for the DQD in semiconductor structures. We consider the DQD formed in a two-dimensional electron gas by gate voltages in nearby electrodes. Our model is quasi-two dimensional, where the vertical direction is under strong confinement and the corresponding motion along it is considered quantized and frozen. The remaining planar problem is solved for the presence of two electrons in a potential profile of a DQD. Spin-orbit interactions, with the Rashba and Dresselhauss contributions, are included in our calculations, as is the effects of applied fields, electric and magnetic. The eigenstates are calculated by a modified split-operator method [1] to include two electrons and their spins. The energy levels are obtained in this method without the use of basis states, and in this way the method can give improved results for states close to a continuum of states. We present the dependence of the energy levels for applied magnetic and electric fields, aiming t use them in future simulations of the dynamics under time-dependent field for quantum computation applications. [1] M. H. Degani and M. Z. Maialle, J. of Computational and Theoretical Nanoscience 7, 454 (2010).
 
ID 77
Wave-Packet Dynamics in Quantum Spin Hall Systems
Marcos Henrique Lima de Medeiros
Universidade de São Paulo
In the context of research on topological phases of matter, quantum spin Hall (QSH) systems are specially interesting for applications in spintronics. In this work, we numerically study the dynamics of wave packets in mercury telluride (HgTe) quantum wells. This system presents topologically-protected, spin polarized edge states (a signature of the QSH effect) if the width of the well is larger than a critical value (d > d_c ~ 6.3 nm). In our simulations, we consider the evolution of Gaussian wave-packet using the BHZ Hamiltonian using the method of (Fourier) split-operator method. We observe oscillatory behavior of the mean position of the packet that is closely related to effect of zitterbewegung. We also study the spin separation of the package into edge-state-like patterns at the edge of the system. The strong dependence of these behaviors with the initial conditions and the presence of electrical fields (in/out-of-plane) is also discussed.
 
ID 78
Investigation of transport properties on PbTe/Pb1-xSnxTe heterostructures
Sandra Nakamatsu, Marcelos L. Peres, Demétrio A. W. Soares, Anderson K. Okassaki, Celso I. Fornari, Paulo Henrique O. Rappl, Eduardo Abramof
Universidade Federal de Itajubá, Instituto Nacional de Pesquisas Espaciais
PbTe compounds have been used for the development of infrared photodetectors and diode lasers [1] over the decades. Introduction of Sn atoms makes this material even more interesting for practical applications as well as from the basics physics point of view. According to the band inversion model, the gap of Pb1-xSnxTe decreases as Sn composition increases, and vanishes for an intermediate alloy composition. Further increasing of Sn concentration leads to the band inversion and the energy gap starts to increase up to the SnTe value. Very recently, it was discovered that in the region of band inversion, transition from metallic to crystalline topological insulator (TCI) occurs [3]. Recent theoretical work demonstrated that the PbTe/Pb1-xSnxTe heterostructures can present topological states in the interface of the heterojunction. Such an interface of PbTe and Pb1−xSnxTe, at which four Dirac cones appear, is analogous to the surface of a weak TI [4]. In this work we perform electrical characterization in PbTe/Pb1-xSnxTe films for different values of x close to the band inversion in order to verify the existence a single gapless helical state in the [111] direction at the heterostructure interface. Morfological characterization will also be performed in order to provide a detailed view of these new structures. We hope that this work contribute to a better comprehension of the nature of topological insulators based on IV-VI compounds. Acknowledgments: The authors would like to acknowledge CAPES and FAPEMIG for support. References [1] I. U. Arachchige and M. G. Kanatzidis, Nano Lett. 9 (4), 1583 (2009), [2] R. Jaramillo et al, Jour. Appl. Phys. 119, 035101 (2016), [3] P. Dziawa, Nature Materials 11, 1023 (2012)
 
ID 79
Transition from negative to positive photoconductivity in $p$-type $Pb_{1-x}Eu_{x}Te$ films
Marcelos L. Peres, Marília de J. P. Pirralho, Demétrio A. W. Soares, Anderson K. Okasaki, Celso I. Fornari, Paulo Henrique de O. Rappl, Eduardo Abramoff
UNIFEI, INPE
The phenomenon of photoconductivity has been used as an important tool to investigate the presence of additional states within the band structure of semiconductors [1] and has provided basic knowledge that allowed the development of photodetector and sensor devices along the last decades [2]. We investigated the photoconductivity effect in $p$-type $Pb_{1-x}Eu_{x}Te$ films for x=0.01, 0.02, 0.03, 0.05 and 0.06 at T=300K. The measurements revealed a clear transition from negative to positive photoconductivity as the Eu content x is increased at room temperature. This transition is related to the metal-insulator transition that occurs due to the disorder originated from the introduction of Eu atoms and it is an Anderson transition. Our investigation found that, from the potential application point of view, the sample x=0.06 is more suitable, i.e. it presents an almost noise free signal and the higher photoconductivity amplitude response observed. The photoconductive for the sample with x=0.06 was further investigated in the temperature range of 77 – 300K and, surprisingly, multiple additional transitions were observed with amplitudes that reached around 200 times the original value before illumination. We show that this anomalous behavior is a consequence of the generation and recombination rates between the bands and the 4f  level and a defect level located inside the bandgap. The main goal of this work is to make a prospective study of NPC effect in $p$-type $Pb_{1-x}Eu_{x}Te$ films in both sides of the metal-insulator transition aiming to possible new applications in the development of photonic devices that operate from low temperature up to room temperature. Acknowledgments: The authors would like to acknowledge CAPES and FAPEMIG for support. References: [1] M. M. Furchi et al, Nano Letters 14, 6165-6170 (2014). [2] R. Jaramillo et al, Jour. Appl. Phys. 119, 035101 (2016)
 
ID 80
Engineering the electron g-factor tensor with tunnel-coupled quantum wells
Marcelo A. Toloza Sandoval, Erasmo Assumpção de Andrada e Silva, Jhon Elber Leon Padilla, Antonio Ferreira da Silva, Giuseppe Carlo La Rocca
Instituto de Física de São Carlos - Universidade de São Paulo, Instituto Nacional de Pesquisas Espaciais, Instituto de Física - Universidade Federal da Bahia, Scuola Normale Superiore and CNISM

Aiming to provide new parameters to use in the electron g-factor engineering and spintronic applications, we investigate the interplay between quantum tunneling and structure inversion asymmetry (SIA) effects on the electron g-factor anisotropy in tunnel-coupled semiconductor quantum wells (QWs). Since the g-factor anisotropy sign change in narrow wells is due to SIA and determined by the electron average position, in double QW structures the quantum tunneling effect plays a pivotal role established by the central barrier length, which stands for the coupling parameter between the wells. Due to the quantum confinement, the anisotropic g-factor renormalization is take into account within envelope-function theory based on the Kane model for the bulk, deriving the effective-mass Hamiltonian for the electronic states in III-V QWs in the presence of an external magnetic field in the both fundamental configurations, i.e., in the QW plane and along the growth direction. Using first order perturbation theory, the corresponding components of the electron g factor tensor are analytically calculated, as a function of the QW width and central barrier length, for symmetric and asymmetric tunnel-coupled InP/InGaAs QWs. Results for single and noninteracting QWs are exactly reproduced as limit cases.

ID 81
Prediction of quantum spin hall insulators using machine learning: trends in the chemical space
Carlos Augusto Mera Acosta, Christian Carbogno, Runhai Ouyang, Luca M. Ghiringhelli, Adalberto Fazzio, Matthias Scheffler
Fritz Haber Institute of the Max Planck Society - Berlin, Instituto de Física - Universidade de São Paulo
Quantum spin hall insulators have attracted considerable scientific interest in recent years [1]. The search for new TIs has often focused on elements with a strong spin-orbit coupling (SOC) [2], which can induce the necessary topological transition. In this work, we have computed the topological invariant Z2Z2 for 200 functionalized honeycomb-lattice systems using our recent Wannier center of charge (WCC) implementation in the {FHI-aims} electronic structure code [3]. Besides confirming the TI character of well-known materials, e.g., functionalized stanene, our study found several other yet unreported TIs. This reveals that also elements with relatively low SOC can form TIs. To understand the calculated results and enable high-throughput prediction, we carried out descriptor identification for the WCCs by a newly developed machine learning approach based on compressed sensing [4]. A physically interpretable descriptor as function of only atomic parameters of the material consituent atoms was found, and many new TIs were predicted.
This work received funding from The Novel Materials Discovery (NOMAD) Laboratory, a European Centre of Excellence.
 
 
ID 82
Valley filtering in graphene due to substrate-induced mass potential
Diego Rabelo da Costa, A. Chaves, G. A. Farias, F. M. Peeters
Universidade Federal do Ceará, University of Antwerp
The unique band structure of graphene has brought the possibility of developing devices based on different degrees of freedom, using its different pseudo-spin states (pseudo-spintronics) and electronic valleys (valleytronics). Valley filtering in graphene has been pursued by many researchers, as a path to use the valley degree of freedom of electrons as the basis for a future valley-tronics [1]. The interaction of monolayer graphene with specific substrates may break its sublattice symmetry and results in unidirectional chiral states with opposite group velocities in the different Dirac cones [2]. Taking advantage of this feature, we propose a valley filter based on a transversal mass kink for low energy electrons in graphene, which is obtained by assuming a defect region in the substrate that provides a change in the sign of the substrate-induced mass and thus creates a non-biased channel, perpendicular to the kink, for electron motion. By solving the time-dependent Schrödinger equation for the tight-binding Hamiltonian, we investigate the time evolution of a Gaussian wave packet propagating through such a system and obtain the transport properties of this graphene-based substrate-induced quantum point contact. Our results demonstrate that efficient valley filtering can be obtained, provided: (i) the electron energy is sufficiently low, i.e. with electrons belonging mostly to the lowest sub-band of the channel, and (ii) the channel length (width) is sufficiently long (narrow). Moreover, even though the transmission probabilities for each valley are significantly affected by impurities and defects in the channel region, the valley polarization in this system is shown to be robust against their presence. References: [1] J. R. Schaibley, et al. Nat. Rev. Mater. 1, 16055 (2016), [2] M. Zarenia, et al. Phys. Rev. B 86, 085451 (2012).
 
ID 83
A theoretical search of the optimal exact exchange fraction for Phosphorene
Eduardo Santos Carvalho, Marília Junqueira Caldas
USP
Phosphorene is a new material that has recently been synthesized from Black Phosphorus (BP) by exfoliation (1) or assisted plasma techniques (2). The structural characteristics of Black Phosphorus and Phosphorene are strongly reminiscent of graphite and graphene (3), with atomic layers of hexagonal symmetry stacked one above the other. A large number of interesting properties and applications of Phosphorene have already been announced and reported, such as the intrinsic semiconductor nature and presence of direct gap (4), highly promising for optical applications. We here present a study of Phosphorus-based systems using Density Functional Theory DFT, focusing on the choice of hybrid (incorporating a fraction of exact exchange) PBEh functional for Phosphorene. Given that the optimal exchange fraction can be dependent on the studied system [5], calculations were made from small molecules involving Phosphorus, as well as concentric and increasing hexagonal rings (Hydrogen-saturated) from which extrapolation to Phosphorene could be obtained. To do that, we use combined PBEh with quasi-particle GW corrections. We find that the exact exchange fraction needed in these systems decreases with the percentage increase of the phosphorus atoms in relation to those of hydrogen, and in the concentric rings increasing converges to 54.5%. 1 CASTELLANOS-GOMEZ, A. et al. 2D Materials, IOP Publishing, v. 1, n. 2, p. 025001, 2014. 2 LU, W. et al. Nano Research, Springer, v. 7, n. 6, p. 853–859, 2014. 3 GEIM, A. K.; NOVOSELOV, K. S. Nature materials, Nature Publishing Group, v. 6, n. 3, p. 183–191, 2007. 4 LI, L. et al. Nature nanotechnology, Nature Publishing Group, v. 9, n. 5, p. 372–377, 2014. 5 JR, M. P. et al. Physical Review B, APS, v. 92, n. 19, p. 195134, 2015.
 
ID 84
Effects of light, temperature and magnetic field on the carriers transport in p-InP single nanowire device
Fernando Maia de Oliveira, Edson Rafael Cardozo de Oliveira, Gilmar Eugenio Marques, Adenilson José Chiquito, Marcio Daldin Teodoro
Universidade Federal de São Carlos - UFSCar
In this work, transport properties of carriers in two different semiconductor devices, built from indium phosphide (InP) single nanowires were studied, evaluating the use of zinc as a dopant. Nanowires were synthesized by a Vapour-Liquid-Solid method and the Au/(p-)InP metal/semiconductor contacts were built by conventional photolithography. The investigation of carrier transport phenomena exhibited a Schottky behavior with dependence on light excitation power, temperature and magnitude of the applied magnetic field. The results suggest that Zn incorporation induces localized states, generating acceptor levels near the semiconductor valence band edge. Thus, stable and optically stimulated currents were measured in Au/p-InP device, exhibiting a strong dependency with light excitation power, at both low (3.8 K) and high (300 K) temperatures. A semiconductor behavior was also observed showing a decreasing resistivity with increasing temperature. Effective barrier heights behavior was investigated and associated with the acceptor levels displacements along temperature variation, which linear fit exhibited values of tunneling factor on the order of meV/K at the Au/p-InP interface. A temperature dependent positive magnetoresistance was observed under magnetic field. The linear fit of the square dependence between magnetoresistance and magnetic field intensity allowed verifying the carrier mobility to be approximately constant, under weak magnetic field and low temperature conditions, exhibiting values one hundred times higher than those expected for intrinsic InP nanowires. Such results suggest the great applicability of these semiconductor nanowire devices, mainly acting in photosensitive circuits, such as solar cells, photodetectors and sensors.
 
ID 85
Electronic excitations of the two electron gases confined in the GaAs quantum wells separated by the AlGaAs barrier via resonant Raman spectroscopy
Leonarde do Nascimento Rodrigues, M. J. V. Bell, Mohamed Henini, V. Anjos 
Lab. de Espectroscopia de Materiais - Dep. de Física - Universidade Federal de Juiz de Fora, School of Physics and Astronomy - Nottingham University - United Kingdom
The inelastic light scattering has been widely used in the study of the semiconductor materials and it has become an indispensable technique for the understanding of fundamental physical processes. The effects of the electronic interactions on quantized electronic systems as two-dimensional electron gas are investigated through means of Raman scattering which allows understand the nature of collective excitations which are known as charge density excitations (CDE) and spin density excitations (SDE). According to the classical Raman selection rules in Fermi liquids it is possible to observe two types of electron gas elementary excitations which has dependence on a polarization of light. CDE is active when the laser energy is resonant with a semiconductor optical gap (near resonance regime) and the incoming and outgoing light polarizations are parallel to each other and SDE has incoming and outgoing light polarizations perpendicular to each other. A model developed in this work observes that when the incident laser light is extreme resonant with an optical gap of the material the collective excitations splits into two contributions: a set of renormalized excitations and a non renormalized one with respect to the bare electronic transitions. The latter is known as single-particle excitations (SPE). The purpose of this work is to study double quantum wells. The authors gratefully acknowledges CAPES, CNPQ and FAPEMIG, Brazil, for financial support.
 
ID 86
Overgrowth study of back-bonded III-V semiconductor membranes
Leonarde do Nascimento Rodrigues, J. Garcia Jr., S. Filipe Covre da Silva, Fernando Iikawa, Odilon D. D. Couto Jr, S. L. Morelhão, Ch. Deneke
Laboratório Nacional de Nanotecnologia (LNNano), Instituto de Física Gleb Wataghin - Universidade Estadual de Campinas, Departamento de Física Aplicada - Universidade de São Paulo (USP)

In this work, we investigate the overgrowth behavior of a virtual substrate based on a completely released, wrinkled and in-place bonded GaAs/InGaAs/GaAs membrane. The virtual substrate was obtained by molecular beam epitaxy (MBE) growth of the heterostructure on an AlAs sacrificial layer over a GaAs (001) substrate. In a second fabrication step, the GaAs/InGaAs/GaAs structure is release by selectively removing the AlAs sacrificial layer, cleaned and reintroduced into the MBE, where it serves as template for growth. After atomic hydrogen cleaning, we deposited 10-nm thick InxGa1-xAs layers varying the Indium content from x=0.05 to x=1. Samples are characterized using atomic force microscopy, scanning electron microscopy, 3D reciprocal space mapping at gracing incident x-ray diffraction and photoluminescence measurements. Results from microscopy shows a flat InGaAs layer growth (up to x=0.4) on the membrane, whereas layers on GaAs already show island and dislocation formation at x > 0.3. Furthermore, we observe the formation of bubbles in the membrane for higher In content as well as preferred material migration and accumulation on top of wrinkles. The shift in the critical thickness for island formation is associate to the change in the lattice parameter between virtual substrate and GaAs. This assumption is strongly supported by the x-ray diffraction experiments. Defect free growth is confirmed by transmission electron microscopy of a x=0.1 sample. In order to demonstrate the ability to grow active structures on membranes, we deposited a nominally unstrained InAlGaAs/InGaAs/InAlGaAs quantum well on top of a released wrinkled membrane. As a consequence of a red shift photoluminescence signal from this quantum well compared to a reference grown on GaAs (001) wafers, we have a strain release of the quantum compared to structures grown on GaAs. This results indicates an optical structure with great technological importance. Acknowledgments: CAPES, CNPQ and FAPESP.

ID 87
Spin-orbit coupling effects in nanowires using multiband k.p method: zinc-blende InSb and wurtzite InAs
Tiago de Campos, Paulo Eduardo de Faria Junior, Martin Gmitra, Guilherme Matos Sipahi, Jaroslav Fabian
Instituto de Física de São Carlos - USP, Institute for Theoretical Physics - Regensburg University
We perform comprehensive numerical calculations of spin-orbit effects in semiconductor nanowires. In particular, we focus on zinc-blende InSb and wurtzite InAs semiconductors, and employ realistic k.p models fitted to first-principles band structures [1], to obtain spin-orbit spin splittings of the electronic subbands for square, circular, and hexagonal nanowires. In addition to the bulk-inversion asymmetry spin-orbit fields (Dresselhaus in zinc-blende and Rashba in wurtzite phases), we also apply a transverse electric field to induce Rashba spin splittings caused by structure inversion asymmetry. We fit the numerical band structures to symmetry-based effective Hamiltonians and obtain important materials parameters for the lowest subbands, including the spin-orbit spin splitting magnitudes and spin textures. Our work is important in the current research related to Majorana states in semiconductor nanowires. [1] FARIA JUNIOR, P. E. et al. Realistic multiband k.p approach from ab initio and spin-orbit coupling effects of InAs and InP in wurtzite phase. Physical Review B 93, 235204 (2016)
 
ID 88
Ozone gas sensing on Zn0.95Co0.05O thin films grown by spray pyrolysis
Marcio Peron Franco de Godoy, Y. J. Onofre, L. F. da Silva, A.C. Catto, V.R. Mastelaro
Universidade Federal de São Carlos - UFSCar, Instituto de Física de São Carlos - USP
Zinc oxide thin films are strategic systems for high power electronics and transparent electrodes due to its wide bandgap. Nowadays metal transition doping becomes interesting to introduce magnetic properties and Co-doped ZnO have been recognized as a promising material in the field of spintronics. Recently ZnCoO nanostructured films were employed as gas sensors due to its chemical sensitivity to harmful gases as ozone (O3). Electrical measurements showed an enhancement on sensor resistance in the presence of Co even at low-level concentrations. However the doping processes in oxides are strongly dependent on synthesis method. Spray pyrolysis is a large scale production method with advantages as low-cost and versatility when considered as alternative route to produce thin oxide films. Our work shows the investigation of Co-doped ZnO films employing zinc acetate dihydrate and cobalt acetate dissolved in distilled water in a low molarity 4 10-3. The structural properties of Zn0.95Co0.05O films with three different thicknesses were investigated by X-rays diffraction (XRD) and scanning electron microscopy (SEM). ZnCoO films grown on (001) silicon substrates present wurtzite phase with high directional preference for the (002) direction as compared to films grown on amorphous glass substrates. Crystallite size varied in the range between 15-25nm, increasing as thickness increases. The electrical response to ozone was measured into a chamber which allows the control of substrate temperature and ozone concentrations. The response was considered as the ratio between the electrical resistance when the device is exposed to an ozone content air flux and pure air at temperatures above 200oC. The response of devices for thicker film (~400 nm) was around 2 for 260 ppb of O3 while a high response is achieved for thinner film (~50 nm) at the same O3 concentration. Considerations about the electrical resistance and the role of thicknesses are discussed.
 
ID 89
Parallel transport in PbTe/PbEuTe quantum wells
Fernando Silva Pena, Marcelos Lima Peres, Marília de Jesus Pascoa Pirralho, Demétrio Artur Werner Soares, Paulo Henrique de Oliveira Rappl, Eduardo Abramof 
Universidade Federal de Itajuba, Instituto Nacional de Pesquisas Espaciais
PbTe based quantum wells (QW´s) has have been widely used to fabricate infrared (IR) lasers, IR detectors and thermogenerators [1]. Also, due to the large interest in spintronic devices developments, some efforts have been actually dispended to the investigation of spin-orbit (SO) coupling and quantum Hall effect in different nanostructures systems and PbTe based structures have emerged as potential candidates for the development of spintronic devices [2]. We investigated the photoconductivity effect in n-type PbTe/Pb1-xEuxTe quantum wells (QW´s) for temperature range of 300K to 77K using infrared light. The measurements revealed that at high temperatures the photoresponse has small amplitude. As temperature decreases it reaches a maximum amplitude around 100K which is 1000 times higher than the original before illumination. Unexpectedly, for further reducing of temperature, the amplitude starts to decrease. This behavior indicates the presence of a transition regime which is a result of parallel transport that occurs between the barriers and the QW even if the barriers are insulators. For temperatures below 100K, the transport is more effective in the QW where the signal decreasing can be associated to the electron-electron scattering due to the increasing of carrier concentration that occurs under illumination. For a further analysis, we performed Hall measurements under dark and light conditions and a general picture of the physical process is presented. We were also able to investigate the persistent photoconductivity effect which is a result of defect states present within the band structure of the QW and barriers. We hope that this investigation leads to the improvement of infrared sensor devices based on PbTe structures. [1] Pei, Y. L et al, Journal of Alloys and Compounds 514 (2012). [2] B. Grbic, et al, Phys. Rev. B 77, 125312 (2008).
 
ID 90
Silicon Surface Modification: Characterization by SEM, IES and RBS
Danilo Roque Huanca, Rosimara Passos Toledo, Carlos Eduardo Silveira Dias, Walter Jaimes Salcedo
Universidade Federal de Itajubá, Universidade de São Paulo
Crystalline silicon (c-Si) surfaces were modified by depositing aluminum (Al) onto its polished surface and then was sintered at 500 °C in inert atmosphere for different times. The Al behavior after this thermal process was investigated as function of the sintered time by Rutherford backscattering (RBS), scanning electron microscopy (SEM), four-point method, and impedance electrochemical spectroscopy (IES). The results shown that depending on the annealing time, the electrical sheet resistance of these samples increases its value in exponential way, so that for 8.0 hours it is about four times larger than that observed in untreated c-Si. According to SEM and RBS analysis, this behavior seems to be linked to the presence of defects at the surface in which Al deposited. These defects appears as pits and nanoparticles, possibly composed by aluminum silicates or even Al-Si oxides. The IES analysis shows us that this treatment promotes not only the Al diffusion, as pointed by RBS, but also the formation of a high resistive layer (HRL) from the surface toward the c-Si bulk. The wide of the layer is sintering time dependent. According to the RBS profile, this HRL is not homogeneous in composition, so its dielectric permittivity also is not; hence, it can not be modeled having a typical capacitance and resistance. However, in a first approach we can do. The effect of this treatment upon the depletion layer capacitance is increase its value, as well as of the double layer capacitance. Although this layer is hard and high resistive, the experience made by immersing them solution composed by 48% HF and 70% HNO3 (1:9) reveals that it is prone to the HF attack, then the etching rate of them increases as function of the annealing time, but this relationship is not linear showing a maximum etching rate peak for the simple annealed by 2.0 hours. In summary, the treatment of silicon by deposition of Al upon its polished surface followed by annealing enhances the sheet resistance
 
ID 91
Ab initio Results for the Structural and Electronic Properties of Intrinsic Defects in PbTe
H. W. Leite Alves, S. M. Ladislau, P. D. Borges, J. E. Petersen, L. M. R. Scolfaro
Universidade Federal de São João del Rei, ICET - Universidade Federal de Viçosa, Texas State University
Thermoelectric devices (TD) have great promise in dealing with the challenges of the growing demand for alternative clean energy and Te-based materials well-known candidates for them. Recently, we have shown that the high values for the dielectric constant, together with anharmonic LA-TO coupling, reduces the lattice thermal conductivity and enhances the electronic conductivity in PbTe[1]. In the literature, it was shown that, by alloying this material with Se or with Sn, both their electronic conductivity and dielectric permittivity are also enhanced. The remaining question is whether the intrinsic defects in this material play the same role, once that there is a few experimental and/or theoretical data on this subject. So, in this work, we show our results, by using the Density Functional Theory within both the Local Density Approximation, gradient conjugated techniques (VASP code) together with the supercell models, for the structural and electronic properties of vacancies and antisite defects in the rocksalt PbTe. Our obtained results have shown that both the Pb and Te antisites without two electrons are the favorable defects in Pb and Te rich conditions, respectively. Moreover, in the perfect stoichiometry condition, the antisites, as well as the Te vacancy, are equally probable to find, all of them at also in the 2+ charge state. Considering the charge injection in the system, all the defects change from the 2+ charge state to the 2- one within 14 meV, at around 70 meV from the valence band edge. This feature makes it difficult to experimentally characterize these defects in PbTe separately. As a consequence, the defect localized states in the bandgap reduces the electronic conductivity in PbTe, which is not good for the TD devices, once the thermoelectric figure of merit is directly proportional to it. We acknowledge the support from FAPEMIG (grant CEX APQ 02695-14 and PIBIC/FAPEMIG/UFSJ).
[1] H. W. Leite Alves, et al., Phys. Rev. B, 87, 115204 (2013).
 
ID 92
First-principles calculations of the stability, vibrational and dielectric properties of PbSnTe alloys
H. W. Leite Alves, A. R. R. Neto, L. M. R. Scolfaro, J. E. Petersen, P. D. Borges
Universidade Federal de São João del Rei, Texas State University, ICET - Universidade Federal de Viçosa
Group IV Tellurides are formidable functional materials, and lead tellurides are no exceptions. This simple rocksalt-type compound is widely known thermoelectric (TE) materials with excellent performances, among other applications. Recently [1], we have shown that the high values for the dielectric constant, together with anharmonic LA-TO coupling, reduces the lattice thermal conductivity and enhances the electronic conductivity in PbTe, which is good for TE devices. Moreover, it was also shown that by alloying this material with Se, the electronic conductivity of the alloys is also enhanced [2]. But, it is not clear if the same occurs when alloying PbTe with Sn. We show, in this work, our theoretical results for the structural, vibrational and dielectric properties of Pb1-xSnxTe alloys. The calculations were carried out by using the Density Functional Theory, gradient conjugated techniques, the plane-wave pseudopotential method (VASP and abinit codes). The alloys were described by both the Virtual Crystal and the Generalized Quasi-Chemical Approximations. Our results show that, while their lattice parameters obey the Vegard rule, their bulk moduli do not. Based on this feature, we have detected that when increasing the Sn concentration x, the anharmonic LA-TO coupling enhances and reach its maximum for x ~ 0.75. This corresponds to the maximum value for the dielectric constant as well. Consequently, the obtained lattice contribution to the static dielectric constant is higher, when compared with both PbTe and SnTe bulk values, showing that the alloy can behave better as TE device than their bulk counterparts. We acknowledge support from FAPEMIG (grant CEX APQ 02695-14). [1] H. W. Leite Alves, et al., Phys. Rev. B 87, 115204 (2013). [2] Y. Pei, et al., Nature 473, 66 (2011).
 
ID 93
Ab initio results for the electronic, vibrational and dielectric properties of PbTe/SnTe (001) superlattices
H. W. Leite Alves, A. R. A. Viana, P. D. Borges, J. E. Petersen, L. M. R. Scolfaro
Universidade Federal de São João del Rei, ICET - Universidade Federal de Viçosa, Texas State University
Thermoelectric Devices(TD) have promise in dealing with the challenges of the growing demand for alternative clean energy and IV-Te-based materials well-known candidates for them. We have shown recently that the high values for the dielectric constant, together with anharmonic LA-TO coupling, reduces the lattice thermal conductivity and enhances the electronic conductivity in PbTe. It has been shown that the electronic structure of some small PbTe/SnTe superlattices, besides the fact that these nanostructures can work as TD, has the features of a weak topological insulator. However, there is no theoretical data about the vibrational and/or dielectric properties of these nanostructures. In this work, we show our results, by using the Density Functional Theory and gradient conjugated techniques (VASP and abinit codes) together with the supercell models, for the phonon, dielectric and electronic properties of the rocksalt (PbTe)n/(SnTe)m (001) superlattices with several PbTe (n) and SnTe (m) thicknesses. Our obtained results has shown that if n = m = an odd integer, the nanostructure has P4/mmm symmetry and the resulting band structure has the same features of the available theoretical data. On the other side, it has P4/nmm symmetry with the same behavior observed for the HgTe/CdTe superlattices or quantum wells. Based on these remarks, we have deduced that when the SnTe thickness is greater than around 6 nm, the nanostructure starts to behave as a topological insulator due to the character inversion of the top of valence and the bottom of conduction bands. The obtained phonon data have shown that the folding of the acoustic modes reduces the phonon group velocities and thermal conductivities by enhancing the anharmonic LA-TO coupling observed in both PbTe and SnTe bulks. As consequence, the obtained lattice contribution to the static dielectric constant is also enhanced when compared with bulk counterparts. We acknowledge support from FAPEMIG (grant CEX APQ 02695-14).
 
ID 94
Structural and electronic properties of vacancies and impurities in non-passivated silicon nanowires by first-principles calculations
H. W. Leite Alves, F. L. Almeida Cruz, A. C. M. Carvalho
Universidade Federal de São João del Rei
In recent years, silicon nanowires (SiNWs) have been considered as important nano systems for future opto- and electronic devices, e.g., as active media in highly efficient photovoltaic devices or in multigate FETs. In device applications, intrinsic defects, as well as extrinsic dopants, play a significant role. However, almost all published theoretical works on this matter have considered only passivated nanowires. Our recent theoretical results [1] have shown that, for non-passivated SiNWs, the rearrangement of Si atoms at the lateral surface follow the same features of the Si surfaces, forming a buckled dimers array. We have also noted that, at the interface of two dimer arrays, a facet is formed. With these structural features, surface states appear at the bandgap and the SiNWs have a metallic behavior. So, it is interesting to know if a defect, such as vacancy, or an impurity can interact with these surface states, "cleaning" the nanowire bandgap. In this work, by using the Density Functional Theory, gradient conjugated techniques, and the plane-wave pseudopotential method, we have evaluated the structural properties, formation energies and the electronic structure for the Si vacancies as well as for B, C and N impurities in some sites of the SiNWs. Our results are in good agreement with both available theoretical and experimental data. Based on our obtained results, we show that, due to the metallic character of the nanowire, the vacancies and all the impurities considered are stable in the (-2) charge state. The evaluated formation energies indicate that vacancies are created at the facets, while the impurities remain preferentially at the interstitial sites. As a consequence, surface states are filled and a small bandgap appears in the calculated band structures of all studied impurities and defects in SiNWs. We acknowledge support from FAPEMIG (grant CEX APQ 02695-14). [1] A. E. A Furtado, et al., Physics Procedia 28, 67 (2012).
 
ID 95
Valley polarized magneto-optical absorption in MoS$_2$ quantum rings
Leonardo Villegas Lelovsky, Diogenes Oliveira, Fanyao Qu
Departamento de Física - Universidade Federal de São Carlos, Physics Institute - University of Brasília
Due to direct optical transitions in spin-coupled K-valleys, photoluminescence (PL) and magneto-PL of monolayer molybdenum disulphide (MoS22) demonstrate high valley polarization. Using MoS22 quantum rings (QRs), we simultaneously explore the influence of magnetic field, field setup, spatial confinement and the Aharonov-Bohm (AB) quantum-interference effect. Our magneto-optical absorption spectra demonstrate that the optical valley selectivity observed in the monolayer MoS22 is inherited to the QRs. Moreover, it is robust against variation of magnetic flux and of the QR-geometry. However, unlike the monolayer bulk material, the frequency of the light absorbed and absorption intensities manifest themselves in a sizable quantum-size effect. Besides, they can also remarkably be tuned by magnetic field and its setup. The valley-selectivity along with giant tunability of the optical absorption spectrum open up new opportunities for quantum information devices based on valley qubit and integrated valley and spin-tronic devices.
 
ID 96
Anomalous photoconductivity in Bi2Te3 topological insulator films
Marilia Jesus Páscao Pirralho, Marcelos Lima Peres, Fernando Silva Pena, Demetrio A. W. Soares, Anderson K. Okazaki, Celso I. Fornari, Paulo H. O. Rappl, Eduardo Abramof 
Universidade Federal de Itajubá, Instituto Nacional de Pesquisas Espaciais
Topological insulators represent a new state of quantum matter that have an insulating bulk band gap but present metallic surface states. The surface states are topologically protected against non-magnetic impurities and it is possible the existence of a pure spin polarized current [1]. Three dimensional topological insulator as Bi2Te3 has attracted attention due to its topological properties and its potential application for development of spintronic devices [2]. In literature there is little or no information about photoconductivity in topological insulators, in particular in Bi2Te3. Photoconductivity measurements represent a powerful tool to probe the presence of defect states within be band structure and transport via more than one conduction channel. In this work we investigated the photoconductive properties of Bi2Te3 epitaxial layers in the temperature range of 77 to 300K. Unexpectedly, our measurements indicated that samples present negative photoconductivity, where the conductivity reduces under illumination, in the whole range of temperatures. In addition, these measurements revealed the presence of persistent photoconductivity effect for low temperatures, 77K-170K, which may be associated to the existence of a defect level within the band gap. From the photoconductivity decay curves, when light is removed, we could extract recombination times as a function of temperature and hence extract the energy associated to the traps located in band gap[3]. This study will reveal the effect of disorder in the photoconductivity properties in Bi2Te3 films and the role of surface states in the negative photoconductivity effect. Acknowledgments: The authors would like to acknowledge CAPES and FAPEMIG for support. References: [1] P. Dziawa, et al, Nature Materials 11, 1023 (2012), [2] Y. L. Chen, et al. Science 325, 178 (2009), [3] J.A.Hagmann, X.Liu, M.Dobrowolska and J.K.Furdyna. Journal of Applied Physics, 113: 17C724., 2013
 
ID 97
Thermodynamic properties of antiferromagnetic lattices models via Landau Theory and numerical methods
Renan Maciel, Gerson J. Ferreira
Universidade Federal de Uberlândia
An interesting characteristic of antiferromagnetic lattices is the frustration phenomena. This effect describes the situation in which the spins in a lattice cannot be oriented to completely satisfy all nearest-neighbor interactions. In this context, the minimum energy of whole system does not correspond to the sum of the locally minimized energy. It is also observable that some geometric rearrangement for the spins in the lattice brings a degeneracy to the fundamental state. It can be verified when frustration occurs, for instance, in triangular, face centered cubic, Kagomé, and hexagonal closed packet (hcp) lattices. This phenomenon reveals non-trivial states of matter (e.g. ice spin, liquid spin, spin glass). In addition, antiferromagnetic materials are interesting for spintronic, due to their weak magnetic interaction with the environment. Here we will present our initial studies of these materials, where we analyze its thermodynamic properties. First, we discuss the basic theoretical tools for describing phase transitions using Landau theory. It is one of the most useful tools in condensed matter physics, providing a tangible description of the nature of phase transitions between ordered and disordered states. Second, we apply the Landau theory to study the Ising model, and discuss antiferromagnetic lattices via the XY model. For Ising Model we also use the Metropolis-Hastings to draw samples from a probability distribution and obtain an antiferromagnetic state of the system.
 
ID 98
Study of the electrical properties of Bi2Te3 and Bi2Se3 samples
Wellington Viana da Silva, Valmir A. Chitta, Xavier Gratens
Instituto de Física - USP
In this work we have studied the electrical properties of narrow gap semiconductor samples. The proposed materials were Bi2Te3 and Bi2Se3 that have already been investigated in the past because they have a high thermoelectric efficiency, and are currently studied due to the presence of surface metallic states. These states have been observed in materials called topological insulators that are among the most important recent discoveries of physics due to its high potential of application in the development of the Spintronics [1]. These materials have non-trivial characteristics, which includes an insulating behavior in the bulk and a conductive state on the surface. Both, Bi2Te3 and Bi2Se3 were theoretically [2] and experimentally [3,4] identified as topological insulators. The presence of topological states can be identified experimentally through magneto-transport measurements, where effects of weak localization/antilocalization at low magnetic fields and a large linear magneto-resistance and Shubnikov-de Haas oscillations in the region of high magnetic fields should be observed. Following that, we have performed an electrical characterization of our samples as a function of temperature using resistivity and Hall measurements. Magneto-transport studies were also carried out to verify the possible existence of linear magneto-resistance behavior, localization/antilocalization effects and Shubnikov-de Haas oscillations. In addition, the effect of temperature on magneto-transport properties was verified. References [1] M. Z. Hasar and C. L. Kane, Reviews of Modern Physics 82, 3045 (2010). [2] H. Zhang, Nature Physics 5, 438 (2009). [3] S. Barua et al., Journal of Physics: Condensed Matter 27, 015601 (2015). [4] Y. S. Kim et al., Phys. Rev. B 4, 073109 (2011).
 
ID 99
Microwave induced edge transport in two-dimensional system: comparison with experiment.
E. V. Levinson, A. D. Levin, G. M. Gusev, A. K. Bakarov
Instituto de Física da Universidade de São Paulo, Institute of Semiconductor Physics - Novosibirsk State University
The microwave-induced resistance oscillations and the related phenomenon of zero-resistance states (ZRS) are observed in high-mobility two-dimensional structures under microwave irradiation (MW). There are several theoretical models [1] explaining general features of this phenomenon. However, there are some effects that these models can’t explain consistently. For example, why it is possible to see very strong effect at low MW power; or why ZRS is observed only in ultraclean samples. In our work we observed a nonlocal resistance response to the microwave irradiation in two-dimensional (2D) electron system and made a connection between experimental results and modeling based on the edge state model [2]. Our observations are related to MIRO and ZRS in GaAs/AlGaAs quantum well samples. The measurements were carried out at low temperatures (4.2K) under MW irradiation (frequency range 110 to 170 GHz). We investigated specific sample structure designed for non-local measurements (H-shape). We measured non-local resistance response under MW radiation. To analyze experimental results, we created a 2D model of our structure and calculated all possible trajectories of electrons with different initial conditions under external electromagnetic fields. Simulations showed strong non-local response to microwave radiation consistent with experimental observations. This suggests that edge transport model is appropriate for the studied case. References: 1. I. A. Dmitriev, A. D. Mirlin, D. G. Polyakov and M. A. Zudov, Rev.Mod.Phys. 84, 1709 (2012); J. Inarrea and G. Platero, Phys.RevLett. 94, 016806 (2005); S. A. Mikhailov, Phys.Rev.B. 89, 045410 (2014); Y. M. Beltukov and M. I. Dyakonov, Phys.Rev.Lett. 116,176801 (2016); 2. A. D. Chepelianskii and D. L. Shepelyansky, Phys.RevB. 80, 241308(R) (2009)
 
ID 100
Isolating Majorana fermions with finite Kitaev nanowires and temperature: the universality of the the zero-bias conductance
Vivaldo Leiria Campo Júnior, Luciano S. Ricco, Antonio Carlos Seridonio
Federal University of Sao Carlos, Unesp - Ilha Solteira

The zero-bias peak (ZBP) is understood as the definite signature of a Majorana bound state (MBS) when attached to a semi-infinite Kitaev nanowire (KNW) nearby zero temperature. However, such characteristics concerning the realization of the KNW constitute a profound experimental challenge. We explore theoretically a QD coupled to metallic leads and connected laterally to a topological KNW of finite size at a non-zero temperature and show that overlapped MBSs of the wire edges can become effectively decoupled from each other and the characteristic ZBP can be fully recovered if one tunes the system into the leaked Majorana fermion fixed point. At very low temperatures, the MBSs become strongly coupled similarly to what happens in the Kondo effect with QDs. We present the universal features of the conductance as a function of the temperature and the relevant crossover temperatures. Our findings could offer additional guides to better identify the presence of the Majorana fermion in the system.

ID 101
Influence of Raman laser power density on the optoelectronic properties on multilayers of reduced graphene oxide
Champi Ana, Maria Quintana
Universidade Federal do ABC, Universidad Nacional de Ingieneria - Peru
In this work, the influence of Raman spectroscopy parameters, was studied for different laser power densities on a multilayer system of graphene oxide (MGO), before and after of the soft process of thermal annealing at a temperature of 80 ° C, without involving chemical treatments, allowing the stability of graphene oxides (GO). In this way oxygenated functional groups that the GO presents are retained, verifying the importance of heat treatment on improving the optoelectronic properties of multilayers of reduced graphene oxides (MRGO). I has been found that the Raman laser power density, together with the oxidation degree of MGO influence on energy band gap and on La size of the nanocrystals.
 
ID 102
Localized and free excitons in GaBixAs1-x /GaAs multiple quantum wells
Vanessa Orsi Gordo, M. A. G. Balanta, J. Kopaczek, B. H. B. Santos, A. D. Rodrigues, H. V. A. Galeti, R. D. Richards, F. Bastiman, J. P.R David, R. Kudrawiec, Y. Galvão Gobato
Departamento de Física - Universidade Federal de São Carlos, Faculdade de Ciências Integradas do Pontal - Universidade Federal de Uberlândia,Faculty of Fundamental Problems of Technology -Wrocław University of Technology, Departamento de Engenharia Elétrica - Universidade Federal de São Carlos, Department of Electronic and Electrical Engineering - University of Sheffield
III- V dilute bismide semiconductor material has attracted much attention in the last years due to its potential applications for photonics in near infrared wavelength range and spintronics. Particularly, it was observed a band gap reduction and a large increase of spin-orbit (SO) split-off energy with the increasing of Bi%. This increase of spin-orbit interaction could suppress dominant the non-radiative Auger recombination processes which improves the efficiency of lasers in the near infrared region and is very attractive for semiconductor spintronics as well. On the other hand, GaBiAs alloys have important disorder effects mainly due to potential fluctuations associated to Bi composition variation and formation of clusters which drastically affects the optical emission of this material. Therefore, the investigation of disorder effects is an important issue for potential use of bismides alloys in photonics and spintronics particularly for GaBiAs/GaAs quantum-well (QW) heterostructures, which are relevant for device implementations. In this work, Raman Spectroscopy and magneto-photoluminescence measurements under high magnetic fields were used to investigate optical and spin properties of GaBiAs/GaAs multiple quantum wells (MQWs). Raman results evidences important structural disorder induced by bismuth insertion in MQWs lattices. An anomalous negative diamagnetic shift was observed at higher temperatures and higher laser intensities and was associated to a sign inversion of hole effective mass in GaBiAs structures. In addition, it was observed an enhancement of polarization degree with decreasing of laser intensity (emission dominated by localized excitons). This effect was explained by an important changes of spin relaxation times due to exciton localization by disorder.
 
ID 103
Usual and Unusual Oscillations of Valley Polarized Magnetoexciton and Charged Exciton Absorption in MoS2 Quantum Rings
Fanyao Qu, D. Oliveira, L. Villegas-Lelovsky, A. C. Dias, Jiyong Fu, Rennan Pinheiro, Railson Vasconcelos, José Félix F. de M. Filho
Institute of Physics, University of Brasilia
Monolayer transition metal dichalcogenides (TMDCs) have attracted great interest for fundamental physics such as spin-valley coupled physics, anomalously large binding energies of exciton as well as for the next generation of optoelectronic devices.  Thanks to direct optical transitions in spin-coupled K-valleys, photoluminescence (PL) and magneto-PL of monolayer molybdenum disulphide (MoS2) demonstrate high valley polarization. Using MoS2 quantum rings (QRs), we simultaneously explore the influence of magnetic field, field setup and spatial confinement on the Aharonov-Bohm (AB) quantum-interference effect. Our magneto-optical absorption spectra demonstrate that the optical valley selectivity observed in the monolayer MoS2 is inherited to the QRs. Moreover, it is robust against variation of magnetic flux and of the QR-geometry. However, unlike the monolayer bulk material, the frequency of the light absorbed and absorption intensities manifest themselves in a sizable quantum-size effect. Besides, they can also remarkably be tuned by magnetic field and its setup, e.g., changing the latter may lead to transition from usual AB effect to aperiodic oscillations of magneto-optical absorption spectrum. Finally, the large exciton and trion binding energies in 2D bulk MoS2 are further enhanced in the QRs. Interestingly, the QRs present larger scale of binding energy tunability in comparison with quantum dot. The valley-selectivity along with giant tunability of the optical absorption spectrum open up new opportunities for quantum information devices based on valley qubit and integrated valley- and spintronic devices.
 
ID 104
Novel single-photon source based on two-dimensional monolayer transition metal dichalcogenide quantum dots
Fanyao Qu, A. C. Dias, Jiyong Fu, L. Villegas-Lelovsky, D. L. Azevedo, D. Oliveira, Rennan Pinheiro, Railson Vasconcelos, José Félix F. de M. Filho
Institute of Physics, University of Brasilia
Photonic quantum computer, quantum communication, quantum metrology and quantum optical technologies rely on the single-photon source (SPS). However, the SPS with valley-polarization remains elusive and the tunability of magneto-optical transition frequency and emission/absorption intensity is restricted, in spite of being highly in demand for valleytronic applications. Here we report a new class of SPSs based on carriers spatially localized in two-dimensional monolayer transition metal dichalcogenide quantum dots (QDs). We demonstrate that the photons are absorbed (or emitted) in the QDs with distinct energy but definite valley-polarization. The spin-coupled valley-polarization is invariant under either spatial or magnetic quantum quantization. However, the magneto-optical absorption peaks undergo a blue shift as the quantization is enhanced. Moreover, the absorption spectrum pattern changes considerably with a variation of Fermi energy. This together with the controllability of absorption spectrum by spatial and magnetic quantizations, offers the possibility of tuning the magneto-optical properties at will, subject to the robust spin-coupled valley polarization.
 
ID 105
Third Non-linear Optical Response Of 2D-materials Near Phonon Resonances
Lucas Lafetá, Alisson R. Cadore, Thiago G. Mendes Sá, Kenji Watanabe, Takashi
Tanigushi, Leonardo C. Campos, Ado Jorio, Leandro M. Malard Departamento de Física - Universidade Federal de Minas Gerais, National Institute for Materials Science (NIMS)
In this work we probe the third-order non-linear optical property of graphene, hexagonal boron nitride and their heterostructure by the use of coherent anti-Stokes Raman Spectroscopy. When the energy difference of the two input fields match the phonon energy, the anti-Stokes emission intensity is enhanced in h-BN, as usually expected while for graphene an anomalous decrease is observed. This behavior can be understood in terms of a coupling between the electronic continuum and a discrete phonon state. Moreover, the CARS spectra of (6,5) semiconducting nanotubes will be discussed. We aknowledge financial support from FAPEMIG, CNPq, Finep and CAPES.
 
ID 106
Spin drift and diffusion in one- and two-subband helical systems
Gerson J. Ferreira, Felix G. G. Hernandez, Patrick Altmann, Gian Salis
Instituto de Física - Universidade Federal de Uberlândia, Instituto de Física - Universidade de São Paulo, IBM Research-Zurich

The theory of spin drift and diffusion in two-dimensional electron gases is developed in terms of a random walk model incorporating Rashba, linear and cubic Dresselhaus, and intersubband spin-orbit couplings. The additional subband degree of freedom introduces new characteristics to the persistent spin helix (PSH) dynamics. As has been described before, for negligible intersubband scattering rates, the sum of the magnetization of independent subbands leads to a checkerboard pattern of crossed PSHs with long spin lifetime. For strong intersubband scattering we model the fast subband dynamics as a new random variable, yielding a dynamics set by averaged spin-orbit couplings of both subbands. In this case the crossed PSH becomes isotropic, rendering circular (Bessel) patterns with short spin lifetime. Additionally, a finite drift velocity breaks the symmetry between parallel and transverse directions, distorting and dragging the patterns. We find that the maximum spin lifetime shifts away from the PSH regime with increasing drift velocity. We present approximate analytical solutions for these cases and define their domain of validity. Effects of magnetic fields and initial package broadening are also discussed. We acknowledge support from CNPq, CAPES, FAPEMIG, FAPESP, and the Swiss National Science Foundation.

ID 107
Time-dependent resonant tunneling transport via nonequilibrium Green's functions: a recent look
Mariana Mieko Odashima, Caio Henrique Lewenkopf
Universidade Federal de Uberlândia, Universidade Federal Fluminense
Nonequilibrium Green's functions (NEGF) provide a solid basis for the theoretical understanding of the quantum electronic transport properties. We have recently addressed two nonequilibrium Green's functions approaches for a resonant tunneling structure under a sudden switch of a bias. We have found that the time-dependent Keldysh formulation of Jauho, Wingreen and Meir, and the partition-free scheme of Stefanucci and Almbladh are formally equivalent in the ubiquitous case of wide-band limit and noninteracting electrons, if leads and dot are in equilibrium before the time-dependent perturbation. Although this was an unexpected result, we have developed the explicit formulas of the lesser Green's function and time-dependent current. The present challenge is now to extend NEGF properly to a general couplings and devices. This investigation sheds light on modern practices, which are of great interest to the mesoscopic transport community. [1] A. P. Jauho, N. S. Wingreen, and Y. Meir, Phys. Rev. B 50, 5528 (1994). [2] G. Stefanucci and C. O. Almbladh, Phys. Rev. B 69, 195318 (2004). [3] M. M. Odashima and C. Lewenkopf. Phys. Rev. B 95, 104301 (2017). http://arxiv.org/abs/1611.01459. Acknowledgments: We thank the support of FAPEMIG, FAPERJ, CNPq and CAPES.
 
ID 108
Spin-polarized edge transport in HgTe junctions
Dimy Nanclares, Luis Gregório G. V. Dias da Silva, Leandro R. F. Lima, Caio H. Lewenkopf
Universidade de São Paulo, Universidade Federal Fluminense
Topological insulators (TI) are bulk insulator materials with spin-degenerated metallic edges/surface states. In 2D, the topological behavior manifests itself as the quantum spin Hall (QSH) effect, arising due to a strong spin-orbit coupling and time reversal symmetry. In the QSH regime, the edge states are helical: electrons with different spins travel in opposite directions. Such behavior is a manifestation of a novel quantum state of matter, theoretically proposed in 2006 [1] and experimentally verified on HgTe/CdTe quantum wells in 2007 [2]. Although widely studied in recent years, several questions remain regarding their electronic and transport properties. This work aims to study the spectrum and electronic transport in TIs, using finite-difference [3] and recursive Green's functions methods [4]. The system was modeled by the BHZ effective Hamiltonian [1], obtained by applying the kp method to Dirac's equation and spin-orbit coupling theory. We use a finite differences' method, in which the Hamiltonian was discretized into a lattice, where each site holds four states: electron/hole with spin up/down. Specifically, we have studied the material's local conductance in a set-up with a gated central region, forming a "n-TI-n junction", which have been recently realized in experiments. We find regimes in which the edge states carry spin polarized currents in the TI region even in the presence of a small magnetic field. Moreover, the transition between quantum spin Hall and ordinary quantum Hall effect was analyzed. [1] Bernevig, B. A., Hughes, T. L., Zhang, S. C., Science \texfbf{314} 1757 (2006), [2] Konig et al. Science \texfbf{318} 766 (2007), [3] Scharf,B., Matos-Abiague,A., Fabian,J., PRB \texfbf{86} 075418 (2012), [4] C. H. Lewenkopf and E. R. Mucciolo, J. Comp. Electron. \texfbf{12}, 203 (2013).
 
ID 109
Spin swap of a hole in a wire-embedded artificial molecule with Landau-Zener-Stückelberg-Majorana (LZSM) transitions
Wojciech J. Pasek, Marcelo Z. Maialle, Marcos H. Degani
Faculdade de Ciências Aplicadas da Unicamp
In III-V semiconductor quantum dots the main source of decoherence of confined spin states is interaction of the electron and nuclear spins [1]. The contact term of the hyperfine interaction vanishes for holes due to their p-type wavefunctions [2,3]. In the field of quantum information storage and processing via confined spins this fact suggests using holes rather than electrons [4,5]. We present a configuration interaction study of a InAs quantum molecule formed by application of an electrostatic potential inside an InAs <111> quantum wire. We use the 4-band Kohn-Luttinger Hamiltonian, that introduces mixing of the heavy and light-hole bands. Additionally the Dresselhaus spin-orbit terms for the lowest energy states were included. Both the last mentioned and the non-axial terms in the Kohn-Luttinger Hamiltonian lead to formation of an anticrossing from two levels of opposing spin states. The system can be controlled by time-dependent manipulation of an external electric field applied in the growth direction. Hole can be driven through an anticrossing in one or a few passages, each of which corresponds to a single LZSM transition. We show that a spin swap of a single ground state hole in the system is possible. The process is realized in two stages: by driving the hole through a large tunnelling anticrossing and then by driving the hole through the small spin-mixing anticrossing. The key factor in both operations is the precise control of the parameters of the electric pulses that drive the hole (duration, shape, amplitude, offset). This work was supported by a FAPESP grant. The computation was executed by LaSCADo (FT, Unicamp). [1] F. H. L. Koppens et al., Phys. Rev. Lett. 100 236802 (2008). [2] T. M. Godden et al., Phys. Rev. Lett. 108 017402 (2012). [3] K. De Greve et al., Nature Phys. 7 872-8 (2011).
[4] D. V. Bulaev and D. Loss, Phys. Rev. Lett. 95 076805 (2005). [5] A. Greilich et al., Nature Photon. 5 702708 (2011).
 
ID 110
Spin operations on a double quantum dot using energy avoid crossings
Marcelo Z. Maialle, Justino R. Madureira, Marcos H. Degani
Universidade Estadual de Campinas - Limeira, SP, Universidade Federal de Uberlândia - Ituiutaba, MG
We theoretically investigate the electron spinflip dynamics driven by applied electric fields in a semiconductor double quantum dot (DQD). Our model describes a semiconductor nanowire with local gating to create the DQD potential profile. The two-electron occupation problem is solved including spin-orbit interaction to obtain the eigenstates of the DQD. The dynamics are simulated for gating conditions in which the energy levels of the system present avoid crossings, which are used to obtain a fast spinflip dynamics by Landau-Zener tunneling. An important effect in this condition is the charge cycle through the DQD, which introduces decoherence in the spin dynamics [1]. We have modeled the charge cycle by introducing processes of load and unload of the DQD by electrons in from the source lead, and out to the drain lead. The model utilizes the calculated eigenstates to account for physical aspects of the charge cycle, such as the spin blockade configuration and the charge spatial distribution in the DQD. We demonstrate few operations of the two-electron spin sates by application of electric field pulses. The efficiency of the operations is tested with the inclusion of the decoherence effects of the charge cycle in order to investigate the robustness of the spinflip dynamics via Landau-Zener tunneling. [1] Extreme Harmonic Generation in Electrically Driven Spin Resonance. STEHLIK, J.; et al. Physical Review Letters, v. 112, p. 227601, 2014.
 
ID 111
Effects of Oxygen Contamination on Monolayer GeSe: A computational study
R. Longuinhos, I. S. S. de Oliveira
Universidade Federal de Lavras
Natural oxidation is a common degradation mechanism of both mechanical and electronic properties for most of the new two-dimensional materials. From another perspective, controlled oxidation is an option to tune material properties, expanding possibilities for real-world applications. Understanding the electronic structure modifications induced by oxidation is highly desirable for new materials like monolayer GeSe, which is a new candidate for near-infrared photodetectors. By means of first-principles calculations, we study the influence of oxygen defects on the structure and electronic properties of the single layer GeSe. Our calculations show that the oxidation is an exothermic process, and it is nucleated in the germanium sites. The oxidation can cause severe local deformations on the monolayer GeSe structure and introduces a deep state in the bandgap or a shallow state near the conduction band edge. Furthermore, the oxidation increases the bandgap by up to 23\%, and may induce direct to indirect bandgap transitions. These results suggest that the natural or intentionally induced monolayer GeSe oxidation can be a source of new optoelectronic properties, adding another important building block to the two-dimensional layered materials. The authors acknowledge support from the Brazilian agencies CNPq, FAPEMIG and the computational time spent at LCC-UFLA. R. Longuinhos thanks to J. Ribeiro-Soares, for useful discussions on the manuscript. Reference: PHYS. REV. B 94, 035440 (2016).
 
ID 112
Extrinsic bistability in resonant tunneling diodes
Marcio Daldin Teodoro, Edson Rafael Cardozo de Oliveira, Emerson Chaves dos Santos, Cleyton Alexandre Biffe, Victor Lopez Richard, Gilmar Eugenio Marques, Andreas Pfenning, Fabian Hartmann, Sven Höfling
Departamento de Física - Universidade Federal de São Carlos, Physikalisches Institut, Universität Würzburg, Germany
The electrical bistability phenomenon characterized by an electrical current hysteresis behavior as a function of an applied bias voltage can be observed in different types of quantum system, including resonant tunneling diodes (RTDs), wherein bistability in these kind of nanostructures can be derived both from intrinsic properties or extrinsic system parameters. In this work the effects of bistability were studied by means of an external resistance associated in series with standard GaAs/AlGaAs RTDs. The current versus voltage (i x V) characteristics were investigated as a function of temperature from 10 to 300 K. A clear electrical bistability was observed which area was proportional to the magnitude of the external resistance. In order to understand the temperature effects on the system, the Schulmann’s theoretical approach for RTDs current density behaviour, based on effective mass approximation, was used to fit the electrical i x V experimental results, for different temperatures. The theoretical approach allowed the study of how parameters such as Fermi energy and scattering process affect the Negative Differential Resistance (NDR), showing great agreement with the NDR obtained experimentally.  Since the diode emits light upon voltage conditions due to an ionization impact process, it was also performed electroluminescence measurements as function of applied bias at 10 K under two configurations: with and without 1 k$\Omega$ associated series resistance. Using Kirchhoff’s law for direct comparison, it was found that the resistance associated in series with the RTD allows to access parameters from the high voltage regime, which could not be available without the use of an associated resistance. Therefore, the manipulation of the electrical bistability is an efficient technique for controlling transport properties and light emission of these nanometric devices.
 
ID 113
Effect of Nitrogen incorporation and Thermal annealing on the optical and spin properties of GaPN dilute nitride alloys
M. A. G. Balanta*, H Albalawi, P. B. A. de Oliveira, H. V. A. Galeti, F.Iikawa, M.Henini, C. Cornet, Y. Léger, S. Almosni, A. Balocchi, H. Carrère, Cedric, X. Marie, Y. Galvão Gobato
Departamento de Física - Universidade Federal de São Carlos, Departamento de Engenharia Elétrica - Universidade Federal de São Carlos, School of Physics and Astronomy - University of Nottingham, Instituto de Física Gleb Wataghin- Universidade Estadual de Campinas, UMR FOTON - CNRS, Université de Toulouse, Faculdade de Ciências Integradas do Pontal - Universidade Federal de Uberlândia.
The optical properties of indirect gap GaP compound can be strongly changed by introducing small amount of nitrogen. The coupling of the nitrogen localized states with the conduction band leads to efficient optical emissions. It is thus promise system for optical device applications. In this work, we used several optical spectroscopy techniques to investigate the effect of nitrogen incorporation and thermal annealing on structural, optical and spin properties of GaPN epitaxial layers grown on GaP substrate. The photoluminescence (PL) spectra show broad emission band attributed to the N emission and it shifts to lower energy as the N content increases, which is the typical band gap reduction with the increase of N. Magneto-PL and the temperature dependence of PL results evidence localization of excitons, which come likely from the potential fluctuation of the alloy disorder. Therefore, we expect that increasing N concentration the alloy disorder should increase, consequently the exciton localization effect should also increase, as observed in our optical results. Moreover, we noted that the GaNP alloy layer under thermal annealing improves the optical qualities of the epilayers, increasing the PL intensity and reducing the linewidth, which can be attributed to the reduction of the exciton localization effects. The thermal annealing also increases the spin polarization degree measured under an external magnetic field. Acknowledgments: We acknowledge the Brazilian financial agencies CNPq, CAPES, FAPESP and FAPEMIG. (*) Present address: Faculdade de Ciências Integradas do Pontal, Universidade Federal de Uberlândia, CEP 38304-402,Ituiutaba, MG, Brazil.
 
ID 114
The study of structural properties and magnetic of thin films of ZnO doped with Mn
Bruno de Abreu Silva, Giancarlo Espósito de Souza Brito, Xavier Gratens, Valmir A. Chitta
Universidade de São Paulo
The goal of this work was the production of nanoparticles and thin films of diluted magnetic ZnO, and its structural e magnetic characterization. Ferromagnetic activity in ZnO matrices doped with transition metal ions has attracted great interest of the scientific community because it enables the development of a new physic, called spintronic, which is based on the degree of freedom of the spin of the electrons. The $Zn_{1-x}Mn_{x}O$ alloy seems to be a promising material, since calculations suggested the possibility of ferromagnetism above room temperature [1]. Experimentally different magnetic behaviours have been observed, including ferromagnetism in epitaxial thin films of ZnO [2]. Apparently the magnetic properties of these dilute magnetic oxides depends on the method used in their production. In this work, we use a sol-gel thechnique to produce nanoparticles and thin films of ZnO doped with Mn.The nanoparticles were produces through a lyophilization process followed by a thermal theatment. After that they are dispersed in a solution of N-Butyl alcohol and the films are obtained through dip coating. The samples have been characterized by X-ray diffraction measurements and atomic force microscopy (AFM). In addition, their magnetic properties as a function of magnetic field and temperature have been investigated using a SQUID magnetometer. For the first samples we have produced, AFM results show homogeneous thin films of ZnO with a thickness of the order of 250 nm and nanostructures average size of 40 nm. The results obtained for x-ray diffraction demonstrate intense and well defined peaks, indicating the presence of single phase materials. Magnetic measurements indicate a paramagnetic behavior of the samples, even when they are not doped.  
 
ID 115
Interplay between Andreev and Majorana bound states in a double dot molecule
Joelson F. Silva, Edson Vernek
Universidade Federal de Uberlândia
We present a numerical study of the emergence of Majorana and Andreev bound states a system composed by two quantum dots, in which one of then is coupled to a conventional superconductor, SC1, and the other connects to a topological superconductor, SC2. By controlling the interdot coupling we can drive the system from a two single (uncoupled) quantum dots to a double (coupled) dot system configurations. We employ a recursive Green’s function technique that provides us with numerically exact results for the local density of states of the system. We first show that in the uncoupled dot configuration (single dot behavior) the Majorana and the Andreev bound states appear in an individual dot in two completely distinct regimes. Therefore, they cannot coexist in the single quantum dot system. Then we study the coupled dot configuration. In this situation we show that in the trivial phase of the SC2, the Andreev states are bound to an individual quantum dot in the atomic regime (weak interdot coupling) or extended over the entire molecule in the molecular regime (strong interdot coupling). More interesting features is actually seen in the topological phase of the SC2. In this case, in the atomic limit, the Andreev states appear bound to one of the quantum dots while a Majorana zero mode appears in the other. In the molecular regime, on the other hand, the Andreev bound states take over the entire molecule while the Majorana state remains always bound to one of the quantum dots.
 
ID 116
Rate equation model for an intermediate band solar cell with ratchet states
Anibal Thiago Bezerra, Nelson Studart
Universidade Federal de Alfenas, Universidade Federal do ABC
A semiconductor structure consisting of two coupled quantum wells embedded into the intrinsic region of a p-i-n junction is proposed to be implemented as an intermediate band solar cell with the ratchet state, aiming raise the electron probability of being hit by a second photon escaping into the continuum, and consequently increasing the cell efficiency. The localized conduction subband of the right-hand side quantum well works as the intermediated band, whereas the excited conduction subband of the left-hand side quantum well, coupled to right-hand side one, operates as the photon ratchet state. The photo-excited electrons in the intermediate band are scattered through the thin barrier separating the wells and accumulated into the ratchet subband. A rate equation model for describing the charge transport properties of this solar cell is presented. Calculations are carried out by solving the time-dependent Schrödinger equation within the split-operator method. The efficiency of the current generation is analyzed directly by studying the occupation of the wells subbands in the p-i-n junction, taking into account the charge dynamic behavior provided by the electrical contacts connected to the cell. The current generation efficiency depends essentially from the relations between the optical generation, recombination rates and the scattering rate to the ratchet state. The inclusion of the ratchet states led to both an increase and a decrease in the cell current depending on the transition rates. This suggests that the coupling between the intermediate band and the ratchet state is a key point to be analyzed when developing an efficient intermediate band solar cell. The authors would like to acknowledge Fapemig for the support.
 
ID 117
FTIR and IES Characterization of Fresh and Passivated Macroporous Silicon
Danilo Roque Huanca, Rosimara Passos Toledo, Carlos Eduardo Silveira Dias, Walter Jaimes Salcedo
Universidade Federal de Itajubá, Universidade de São Paulo
As-etched macroporous silicon (MPS) was passivated successful by electrochemical oxidation and then covered by a polyaniline layer by chemical route deposition. The Fourier transform infrared spectroscopy (FTIR)shows the silicon oxide formation within the porous structure and the efficient polyaniline (PANI) deposition. According to it, PANI has been successful deposited in its emeraldine state, which is the most conductive state, and shows that the chemical route is an alternative method and very suitable for depositing PANI in insulating structures, such the oxidized MPS. The electrochemical features of them was investigated by impedance electrochemical spectroscopy (IES) comparing the as-etched and passivated MPS with the crystalline one. In the Nyquist plot for the crystalline silicon (c-Si) it was observed the presence of two semicircles, at high and middle frequency region, respectively. That observed at high frequency region is associated to the effect of the depletion layer capacitance, whereas the second circle, to the double depletion layer. After pore formation, the IES reveals the presence of only one capacitance, which is typical in thick layers. After silicon oxide growth not only yields total resistance of the system arise due to the insulating properties of it, but also the formation of an additional capacitance which associated to the silicon oxide/MPS. After polymer deposition, the presence of one additional capacitance was observed because the PANI/Silicon oxide interface. The total resistance of the system drops to lowest values because the conducting properties of PANI, making the MPS a suitable medium to be used as matrix to host detecting species in chemo- and biosensors. In short, it was shown that electrochemical oxidation is an excellent way to insulate the porous surface, whereas the chemical deposition is excellent alternative way for depositing the PANI films.
The authors thank to the FAPEMIG for the financial support of this research
 
ID 118
Robust helical edge transport at n=0 quantum Hall state
Gennady Gusev, D. A. Kozlov, A. D. Levin, Z. D. Kvon, N. N. Mikhailov, S. A. Dvoretsky
Instituto de Física da Universidade de São Paulo, Institute of Semiconductor Physics Novosibirsk
Two dimensional massless Dirac fermions in the presence of a strong perpendicular magnetic field show several remarkable features that sharply diverge from conventional behaviour. The energy spectrum is organized in Landau levels (LL) with square root versus linear dependence on the magnetic field and square root dependence on the Landau index n versus n+1/2, in comparison with  the parabolic dispersion at the zero field.  The most remarkable consequence of this last property is the existence of a zero-energy Landau level ($\nu= 0$). This is not due to the linear spectrum, but is related to the $\pi$ Berry phase carried by each Dirac point. Therefore, the $\nu = 0$ LL has a magnetic field independent energy, which is quite different from a quantized cyclotron orbit in the conventional quantum Hall effect.  The existence of the zeroth Landau level has been examined by measurements of the integer quantum Hall effect (QHE) in graphene with two-valley degenerate spectrum. Application of other materials that posses a single Dirac cone is of particular interest. Recently a two-dimensional system with a single Dirac cone spectrum, based on HgTe quantum wells, has been discovered. The single spin degenerate Dirac valley allows unambiguous identification of the features resulting from the bulk zeroth Landau level. In addition, the high mobility and giant Lande g-factor favor formation of spin-polarized counter propagating states. In the present paper, we studied the nonlocal transport in 10-probe devices fabricated from HgTe zero-gap quantum structures. We observe a magnetic field induced, giant, nonlocal resistance peak near the CNP in different configurations of current and voltage probes. The nonlocal response is comparable with local resistance and increases rapidly with B. Simple Kirchhoff based estimations and more complicated model calculations clearly confirm the existence of helical edge states originating from the bulk zeroth LL. 
 
ID 119
Radial Heterostructured GaAs/AlGaAs Nanowires
Belarmino Tavares, Marcos Tito, Yuri Pusep
IFSC/USP
Photoluminescence was studied in GaAs/AlGaAs NWs with different radial heterostructures. We demonstrated that manipulation of the emission energy may be achieved by appropriate choice of the shell structure. Structural characterization of the nanowires (NW) was performed with transmittance electron microscopy, photoluminescence and Raman scattering measurements, which unambiguously manifest to the presence of segments crystallized in zincblende and wurtzite phases. Four observed photoluminescence lines are assigned to the radiative recombination of photoexcited electrons confined in the center of the GaAs core and at the heteroboundary between the outer GaAs shell and the inner AlGaAs one with the holes localized at the heteroboundary between the core and the inner AlGaAs shell; both recombinations take place in zinc-blende as well as in wurtzite phases. One additional photoluminescence line is attributed to the spatially indirect recombination between the electrons in zinc-blende and the holes in wurtzite phases. A red shift of the recombination time maximum with respect to the photoluminescence peak energy was demonstrated in doped NWs. The proposed double shell structure with tunneling transparent inner shell sets conditions for easy control of the emission energy of the heterostructured NWs. The results of this study are published in [1-4]. In addition, time-resolved photoluminescence is employed to study electron-hole dynamics. [1] F.E.G.Guimarães, R. A. Caface, H.Arakaki, C.A. de Souza, and Yu.A.Pusep, J.Appl.Phys. 113, 064315 (2013). [2] B. G. Barbosa, H.Arakaki, C.A. de Souza, and Yu.A.Pusep, J.Appl.Phys. 115, 114312 (2014). [3] M. A. Tito, B. G. M. Tavares, H. Arakaki, C. A. de Souza, and Yu. A. Pusep, Superlattices and Microstructures, 100, 918 (2016).
 
ID 120
An interaction potential for ZnSe: A molecular dynamics study
Sandra Cristina Costa Prado, José Pedro Rino
Faculdade de Tecnologia de Mococa, Universidade Federal de São Carlos
Molecular dynamics technique was used to study the structural, thermodynamics, and dynamical properties of zinc selenide, based on a many-body interatomic potential that takes into account two- and three-body interactions. The potential was able to describe the energetic of the zinc-blende, wurtzite and rock salt structures of ZnSe. Knowing the experimental values for bulk modulus, the cohesive energy at experimental density, the effective interatomic potential was parameterized. Other properties, not used in the calibration of the potential, such as the vibrational density of states, is correctly described. Cooling from the liquid an amorphous phase or a re-crystallized material could be obtained. Pair distribution function, coordination number, volume change, and bond angle distributions are presented and compared with experimental available data. The structural phase transition induced by hydrostatic pressure was also studied.
 
ID 121
Synthesis and study of Eu3+ doped luminescent Layered Double Hydroxide (LDH)
Felipe Oliveira Machado, Alysson F. Morais, Ivan G. N. Silva, Danilo Mustafa
Universidade de São Paulo - USP
Rare earth doped, mesostructured double layered hydroxides (LDH) provide significant potential for matching the requirements between molecular and mesoscopic organization of different photon conversion structures and sensitizers. Typically, RE3+ ion absorption bands present low values of molar absorptivity and accordingly exhibit low intensity emission spectra, because the corresponding 4f-4f transitions are forbidden by the Laporte parity rule. This issue can be resolved by exploiting coordinated "antenna" ligands to enhance the initial absorption of photons. Consequent inter-molecular energy transfer to the rare earth ion dramatically increases its quantum emission yield. Anionic sensitizing dyes can directly adsorb to positive charge located on the rare earth atoms leading to an efficient energy transfer, while additional photo-(chemical) conversion systems and/or substrates can be hosted in close vicinity within the mesostructure. This is the key to allow improving the overall energy harvesting efficiency of photo-chemical, -physical and voltaic applications. In this work, we show the design of LDHs with Eu3+, the incorporation of new organic ligand in the interlayer space and the study of their luminescent properties.
 
ID 122
Preparation of luminescent Layered Double Hydroxide (LDH) using memory effect
Alexandre Candido Teixeira, Alysson F. Morais, Ivan G. N. Silva, Danilo Mustafa
Universidade de São Paulo - USP
The structure of layered double hydroxides (LDH) is based on the sheet mineral brucite (MgO). Isomorphic substitution of divalent ions in the neutral brucite sheet with tri or tetravalent ions generates a positive layer charge that has to be compensated by adsorption of anions between the layers. The memory effect in LDHs consist in the recovery of their structure through the rehydration of the material after been submitted to an annealing treatment under specific temperature. By heating the LDH an amorphous oxide phase is formed. The rehydration process can be used to introduce desired anions in the interlayer space of the material, leading to new LDHs. The type of LDH and the annealing temperature are fundamental to the success of the method. The initial material must contain thermally stable anions and the annealing temperature should be carefully controlled to avoid excessive heating which can induce the formation of a stable resistant to the rehydration structure. In principle, the rare earth elements are added to the structure of the material before the thermal treatment. The rehydration of this class of material with sensitizers anions (e.g.1,3,5 benzene tricarboxylic acid) can result in a formation of a luminescent material which can be an alternative to overcome some of the problems encounter in the development of renewable alternative energy sources.
 
ID 123
Topological Degeneracy from Higher Gauge Theory
R. Costa de Almeida, J. P. Ibieta-Jimenez, J. Lorca Espiro, P. Teotonio-Sobrinho 
Instituto de Física - Universidade de São Paulo
Lattice models for topological order obtained from gauge theories have been extensively studied in the literature and are fairly well understood. In recent years, attempts to generalize this situation through, the so called, 2-gauge theories have opened the door to interesting new models and new topological phases which are not described by the previous schemes of classification. In this paper we show that we can go beyond the 2-gauge construction by sacrificing some of the mathematical structure when considering chain complexes of abelian groups. To this aim we employ some machinery from homological algebra which greatly simplifies known constructions for abelian theories. This formalism allows us to systematize a large class of models already available in the literature with the same language and provides an explicit procedure for computing the corresponding topological degeneracies. Finally, a rigorous connection between the ground states and a known cohomology theory is established which leads to a convenient set of quantum numbers with a well-defined physical interpretation.
 
ID 124
Study of Majorana leakage on quantum dots
E. J. P. Biral, J. C. Egues, A. R. Cruz, P. H. Penteado, E. Vernek
IFSC-USP, INFIS-UFU
Majorana Fermions [1] have been extensively investigated in condensed matter systems. In particular, the leakage of Majorana modes into a quantum dot has been studied by considering a setup consisting of spinless [2] and spinfull [3] quantum dots connected to a Kitaev chain. In this work, we look at a spinless quantum dot coupled to two topological nanowires, i.e., Kitaev chains. By using a recursive Green’s function approach we show that it is possible to control the splitting of Majorana modes within the dot by tuning some physical parameters of the system, e.g., the gate voltage. This work is supported by FAPESP, Capes and CNPq. [1] ALICEA, J. New directions in the pursuit of Majorana fermions in solid state systems. Reports on Progress in Physics, v. 75, n.7, p.076501, 2012 .[2] VERNEK, E; PENTEADO, P. H.; SERIDONIO, A. C.; EGUES, J. C. Subtle leakage of a Majorana mode into a quantum dot. Physical Review B, v. 89, n.16, p.165314, 2014. [3] RUIZ-TIJIERINA, D.A.; VERNEK, E.; SILVA, L. G. G. V. D.; EGUES, J. C. Interaction effects on a majorana zero mode leaking into a quantum dot. Phsysical Review B, v. 91, p. 115435, 2015.
 
ID 125
Klein Tunneling on Weyl Cones
João Vitor Ignácio Costa, Denis R. Cândido, J. Carlos Egues
Instituto de Física de São Carlos - USP
The Klein tunneling [1] phenomenon is one of the most counterintuitive results of relativistic quantum mechanics. It is related to the full penetration Dirac particles through high barriers. Here, following Ref.[2], we investigate the associated transmission probability of a single 3D Weyl cone in the context of condensed matter physics, where the effect can be observed easier due the zero effective electronic mass [3]. By applying the appropriated boundary condition between two 3D regions, differing by a barrier potential, we determine the transmission probability. We plot the transmission probability as a function of the electron's incidence angles, noting the formation of transmission rings. These rings are angles of electronic incidence that fully transmit the particles, and which radii depend on the height of the barrier potential V0 . We also propose as a further step the calculation of transmission functions in similar and more complex systems [4][5]. This work was supported by Capes and CNPq. [1] O.Klein, Z.Phys. 53, 157 (1929). [2] Yesilyurt, Can, et al. "Klein tunneling in Weyl semimetals under the influence of magnetic field." Scientific reports 6 (2016). [3] Katsnelson, M. I., K. S. Novoselov, and A. K. Geim. "Chiral tunneling and the Klein paradox in graphene." arXiv preprint cond-mat/0604323 (2006).[4] Okugawa, Ryo, and Shuichi Murakami. "Dispersion of Fermi arcs in Weyl semimetals and their evolutions to Dirac cones." Physical Review B 89.23 (2014): 235315. [5] Bernevig, B. Andrei , et al. "Type-II Weyl semimetal", Nature 527, 495-498. Letter
 
ID 126
Possibilities for layered double hydroxides (LDH) nanotubes in biological applications
Natasha Fioretto Aguero, Alysson Ferreira Morais, Ivan Guide Nunes da Silva, Danilo Mustafa
Institute of Physics - University of São Paulo (IFUSP)
Layered double hydroxides (LDH) are materials formed by the alternating stacking of metallic and anionic layers. These lamellar materials have attracted the attention of the scientific community for the versatility of their composition and the variety of possible morphologies. More specifically, the viability of LDHs in biomedical applications has been demonstrated in the last decade, mainly due to characteristics such as low cytotoxicity and permeability in some types of cell walls. In addition, anionic molecules of biological interest such as DNAs, RNAs or bioactive drugs can be adsorbed on the surface of the LDHs or even protected between the lamellae, which could allow its release in an intracellular environment. One of the major problems when attempting to use interlamellar spacing of LDHs for intercalation of molecules is the restriction imposed by the layer structure of the material. There is, therefore, a limit for the molecule size that can be intercalated. As a solution to this problem, our group proposes structuring LDHs in the form of nanotubes, which considerably increases their surface area and, consequently, their adsorption capacity. This work aims firstly to expose the state of the art in biological applications of LDHs. Next, we wish to demonstrate the potential implications of LDH nanotubes in nanomedicine, focusing on their advantages and the possible challenges that will be imposed in the synthesis and viability of these materials.