I Dia da Correlação Unesp Rio Claro

Transcription

1 I Dia da Correlação Unesp Rio Claro 8:20-8:30 - ABERTURA 8:30-9:30 - Marcos Avila UFABC Simplicity vs. complexity in thermoelectric quantum materials: the cases of FeGa 3 and RT 2 Zn 20 Thermoelectrics is an important part of clean and renewable energy research, and a promising field of applications for many quantum materials. The efficiency with which a thermoelectric device can convert waste heat into useful electricity depends strongly on the electron and phonon transport of the device materials, since it can be quantified as Z = S 2 σ/κ (thermopower squared times the ratio between electrical and thermal conductivities). Thus, contemporary routes towards full optimization of Z often resort to exotic mechanisms that may be tuned to allow a delicate balance between: (i) optimal charge carrier concentration (in the range of semiconductors to semimetals); (ii) minimal electron scattering (typical of simple & crystalline metals); and (iii) maximal phonon scattering (characteristic of complex & glassy structures). Here we highlight some recent in-group and collaborative works that demonstrate such balances between simplicity vs. complexity in two thermoelectric materials of current interest: FeGa 3 with a tetragonal layered structure, and the RT 2 Zn 20 family (R = rare earth, T = transition metal) showing a cage-like structure with 184 atoms per unit cell. Systematic chemical tuning of these systems has resulted not only in the control of desired physical parameters but also given rise to surprisingly rich and unexpected fundamental physics, including quantum criticality. Single crystals with compositions such as Fe 1-x Co x Ga 3 and FeGa 3-y Ge y [1,2]; Y 1-x Gd x Co 2 Zn 20 [3], GdFe 2-x Co x Zn 20 [4] and YbFe 2 Zn 20-y Cd y [5] have been grown and investigated through a variety of bulk and microscopic experimental techniques including resistivity, magnetization, heat capacity, XRD, XAS, ESR, SR and Mossbauer spectroscopy, plus detailed theoretical support from DFT simulations. Analyses of the combined results on these systems, which have allowed the establishment of the mechanisms behind the observed phenomena, will be presented and discussed in the talk within the framework of how thermoelectric materials research can contribute to the understanding of physical phenomena in quantum materials, and vice versa. We thank the Brazilian funding agencies FAPESP, CNPq, CAPES and FINEP for their financial support. [1] J. C. Alvarez-Quiceno et al., Phys. Rev. B 94, (2016). [2] J. Munevar et al., Phys. Rev. B 95, (2017). [3] M. Cabrera-Baez et al., Phys. Rev. B 92, (2015). [4] M. Cabrera-Baez et al., Phys. Rev. B 95, (2017). [5] M. Cabrera-Baez et al., J. Phys.: Cond. Mat. 28, (2016).

2 9:30-10:30 Roberto E. Lagos Unesp Rio Claro The Meaning of Mean Field Theory - Brief History and Applications We present a brief history of Mean Field Theory Approximations (MFTA), starting with Van der Waals, followed by Weiss & Curie, Hartree & Fock, Ginzburg & Landau, BCS, Hubbard and others. As the method is presented we illustrate them with some applications to several well-known Models, namely: Heisenberg, Hubbard, Anderson, BCS, Mott Metal-Insulator Transition, electron-phonon and electron-exciton interactions in condensed matter. MFTA permeates the literature under many particular names: HF, BCS, Effective Medium Approximations, Virtual Crystal Approximation, Coherent Potential Approximation (CPA), the latter in both the single site and cluster (CCPA) versions, and also in a dynamical version (DCPA & DCCPA). Correlation Functions and Green Functions Methods are used throughout. 10:30-11:30 - Eduardo Miranda IFGW "What can happen when superconductivity and antiferromagnetism coexist" The phenomena of superconductivity (SC) and antiferromagnetism (AFM) represent two of the most important phases of strongly correlated systems. They can be fruitfully described by broken U(1) (gauge) and SO(3) (rotation) symmetries, respectively, for which the Landau functional is the key concept. These two phases generally compete with each other and when one is stable the other is generally not. However, there are important physical systems, like iron-based superconductors and heavy fermion materials, in which they can coexist in every crystalline unit cell. This gives rise to interesting secondary phenomena, some of which remain unexplored. I intend to describe some recent results of this rich interplay, namely: (a) the appearance of a secondary triplet superconducting phase when the dominant one is a singlet superconductor, (b) Josephson junctions with a magnetic domain wall, and (c) the concept of Interface SC, which appears only at the interface between materials which are not superconducting in the bulk. 11:30-13:30 LUNCH BREAK

3 13:30-14:30 - André Saraiva UFRJ Giant Atoms and Molecules in Silicon Donors in silicon, conceptually described as hydrogen atom analogues in a semiconductor environment, have become a key ingredient of many "More-than- Moore" proposals such as quantum information processing and single-dopant electronics. The level of maturity this field has reached has enabled the fabrication and demonstration of transistors that base their functionality on a single impurity atom, allowing the predicted single-donor energy spectrum to be checked by an electrical transport measurement. Generalizing the concept, a donor pair may behave as a hydrogen molecule analogue. However, the molecular quantum mechanical solution only takes us so far and a detailed understanding of the electronic structure of these molecular systems is a challenge to be overcome. We provide here a roadmap for modeling silicon nano-devices with one or two group V donors (D). We discuss systems containing one or two electrons, that is, D_0, D^-, D_2^+ and D_2^0 centers [1]. We further present a combined experimental-theoretical demonstration of the energy spectrum of two strongly interacting donor pairs [2,3] and show the first two observations of measurable two-donor exchange coupling. The measured data are accurately matched by results obtained in an effective mass theory incorporating the Bloch states multiplicity in Si, a central cell corrected donor potential and a full configuration interaction treatment of the 2-electron spectrum. Besides the spectrum, the effective mass wavefunction is confirmed through STS experiments [4], calibrating this theory as the standard model for a modern description of impurities in Si. We finally use it to predict the behavior of new devices -- in special we propose an implementation of a Fermi-Hubbard simulator based on STM litographically designed impurity chains. [1] A. L. Saraiva, A. Baena, M.J. Calderón, Belita Koiller, Journal of Physics: Condensed Matter 27, 15 (2015). [2] Juan P. Dehollain, Juha T. Muhonen, Kuan Y. Tan, André Saraiva, David N. Jamieson, Andrew S. Dzurak, Andrea Morello, Phys. Rev. Lett. 112, (2014). [3] M. Fernando Gonzalez-Zalba, André Saraiva, Dominik Heiss, Maria J. Calderón, Belita Koiller, Andrew J. Ferguson, Nano Lett., 2014, 14 (10), pp [4] A. L. Saraiva, J. Salfi, J. Bocquel, B. Voisin, S. Rogge, Rodrigo B. Capaz, M.J. Calderón, Belita Koiller, Phys. Rev. B 93, (2016) [5] Amintor Dusko, Andre Saraiva, Belita Koiller, accepted for NPJ Quantum Information. 14:30-15:30 - Juarez L. F. Da Silva IQSC-USP A comprehensive study of g-factors, elastic, structural and electronic properties of III-V semiconductors using Hybrid-Density Functional Theory Despite the large number of theoretical studies reported for the III-V semiconductors every year, our atom-level understanding is still limited, in particular, due to the limitations of density functional theory (DFT) to yield accurate structural and electronic properties at the same level, which is commonly explained by the

4 unphysical self-interaction problem, which affects mainly the magnitude of the band gap and spin-orbit splitting in semiconductors and oxides. In this talk, we will report a consistent study of the structural and electronic properties of the III-V semiconductors employing the screening hybrid-dft framework, fitting the α parameters for 12 different III-V compounds (AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb) in order to minimize the deviation between the theoretical and experimental values of the band gap and spin-orbit splitting. We obtained α values spreading from to 0.343, which are close to the assumed universal value of Our results for the lattice parameter and elastic constants indicate that the fitting of the α does not affect those structural parameters when compared with the HSE06 functional, where α = Our analysis of the band structure based on the k p method shows that the effective masses are in good agreement with the experimental values, which, in part, can be attributed to the simultaneous fitting of the band gap and SOC. 15:30-16:00 COFFEE BREAK 16:00-17:00 - Herculano Martinho UFABC Desvendando alguns dos mistérios da água confinada: seu segundo ponto crítico A distinção entre gás e líquida desaparece nos arredores e acima de um ponto especial do diagrama de fases de um dado material, chamado de ponto crítico. Em pressões e temperaturas acima deste ponto, diz-se que o sistema como um fluído supercrítico. Os fluídos supercríticos possuem propriedades singulares de solvatação, que os tornam muito importantes para aplicações tecnológicas. De interesse particularmente especial é o comportamento da água em espaços confinados, que se encontra num estado supercrítico. As águas de hidratação um uma enormidade de situações na natureza, como, por exemplo, nas proteínas de nosso organismo, encontra-se no regime confinado. Contudo, apesar a sua reatividade neste regime, suas propriedades termodinâmicas, seu diagrama de fases são um mistério. Por exemplo, previsões teóricas indicam a existência de um segundo ponto crítico para a água confinada. Neste seminário abordaremos recentes avanços obtidos em nosso grupo de pesquisa sobre este tema. Em particular discutiremos estratégias para confinar água, metodologias experimentais e resultados indicando a existência do segundo ponto crítico.

5 17:00-18:00 - Vivaldo Leiria UFSCAR Universal zero-bias conductance in a Majorana-detection setup The zero-bias peak (ZBP) is a strong indication of a Majorana bound state (MBS) when attached to a semi-infinite Kitaev nanowire (KNW) nearby zero temperature. We explore theoretically a QD connected to a topological KNW of finite size and show that at a non-zero temperature regime, the MBS become decoupled from each other if one tunes the system into the leaked Majorana fermion fixed point. Universal aspects of the temperature dependence of the zero-bias conductance are explored to offer additional signatures of the presence of MBS in the device.