Staff directory Jose Hugo García Aguilar

Publications

2018

  • Finite-size correction scheme for supercell calculations in Dirac-point two-dimensional materials

    Rocha C.G., Rocha A.R., Venezuela P., Garcia J.H., Ferreira M.S. Scientific Reports; 8 (1, 9348) 2018. 10.1038/s41598-018-27632-6.

    Modern electronic structure calculations are predominantly implemented within the super cell representation in which unit cells are periodically arranged in space. Even in the case of non-crystalline materials, defect-embedded unit cells are commonly used to describe doped structures. However, this type of computation becomes prohibitively demanding when convergence rates are sufficiently slow and may require calculations with very large unit cells. Here we show that a hitherto unexplored feature displayed by several 2D materials may be used to achieve convergence in formation- A nd adsorption-energy calculations with relatively small unit-cell sizes. The generality of our method is illustrated with Density Functional Theory calculations for different 2D hosts doped with different impurities, all of which providing accuracy levels that would otherwise require enormously large unit cells. This approach provides an efficient route to calculating the physical properties of 2D systems in general but is particularly suitable for Dirac-point materials doped with impurities that break their sublattice symmetry. © 2018 The Author(s).


  • Large spin relaxation anisotropy and valley-Zeeman spin-orbit coupling in WSe2 /graphene/ h -BN heterostructures

    Zihlmann S., Cummings A.W., Garcia J.H., Kedves M., Watanabe K., Taniguchi T., Schönenberger C., Makk P. Physical Review B; 97 (7, 075434) 2018. 10.1103/PhysRevB.97.075434.

    Large spin-orbital proximity effects have been predicted in graphene interfaced with a transition-metal dichalcogenide layer. Whereas clear evidence for an enhanced spin-orbit coupling has been found at large carrier densities, the type of spin-orbit coupling and its relaxation mechanism remained unknown. We show an increased spin-orbit coupling close to the charge neutrality point in graphene, where topological states are expected to appear. Single-layer graphene encapsulated between the transition-metal dichalcogenide WSe2 and h-BN is found to exhibit exceptional quality with mobilities as high as 1×105 cm2 V-1 s-1. At the same time clear weak antilocalization indicates strong spin-orbit coupling, and a large spin relaxation anisotropy due to the presence of a dominating symmetric spin-orbit coupling is found. Doping-dependent measurements show that the spin relaxation of the in-plane spins is largely dominated by a valley-Zeeman spin-orbit coupling and that the intrinsic spin-orbit coupling plays a minor role in spin relaxation. The strong spin-valley coupling opens new possibilities in exploring spin and valley degree of freedom in graphene with the realization of new concepts in spin manipulation. © 2018 American Physical Society.


  • Quantum Hall effect in graphene with interface-induced spin-orbit coupling

    Cysne T.P., Garcia J.H., Rocha A.R., Rappoport T.G. Physical Review B; 97 (8, 085413) 2018. 10.1103/PhysRevB.97.085413.

    We consider an effective model for graphene with interface-induced spin-orbit coupling and calculate the quantum Hall effect in the low-energy limit. We perform a systematic analysis of the contribution of the different terms of the effective Hamiltonian to the quantum Hall effect (QHE). By analyzing the spin splitting of the quantum Hall states as a function of magnetic field and gate voltage, we obtain different scaling laws that can be used to characterize the spin-orbit coupling in experiments. Furthermore, we employ a real-space quantum transport approach to calculate the quantum Hall conductivity and investigate the robustness of the QHE to disorder introduced by hydrogen impurities. For that purpose, we combine first-principles calculations and a genetic algorithm strategy to obtain a graphene-only Hamiltonian that models the impurity. © 2018 American Physical Society.


  • Shubnikov-de Haas oscillations in the anomalous Hall conductivity of Chern insulators

    Canonico L.M., García J.H., Rappoport T.G., Ferreira A., Muniz R.B. Physical Review B; 98 (8, 085409) 2018. 10.1103/PhysRevB.98.085409.

    The Haldane model on a honeycomb lattice is a paradigmatic example of a system featuring quantized Hall conductivity in the absence of an external magnetic field, that is, a quantum anomalous Hall effect. Recent theoretical work predicted that the anomalous Hall conductivity of massive Dirac fermions can display Shubnikov-de Haas (SdH) oscillations, which could be observed in topological insulators and honeycomb layers with strong spin-orbit coupling. Here, we investigate the electronic transport properties of Chern insulators subject to high magnetic fields by means of accurate spectral expansions of lattice Green's functions. We find that the anomalous component of the Hall conductivity displays visible SdH oscillations at low temperature. The effect is shown to result from the modulation of the next-nearest-neighbor flux accumulation due to the Haldane term, which removes the electron-hole symmetry from the Landau spectrum. To support our numerical findings, we derive a long-wavelength description beyond the linear ("Dirac cone") approximation. Finally, we discuss the dependence of the energy spectra shift for reversed magnetic fields with the topological gap and the lattice bandwidth. © 2018 American Physical Society.


  • Spin transport in graphene/transition metal dichalcogenide heterostructures

    Garcia J.H., Vila M., Cummings A.W., Roche S. Chemical Society Reviews; 47 (9): 3359 - 3379. 2018. 10.1039/c7cs00864c.

    Since its discovery, graphene has been a promising material for spintronics: its low spin-orbit coupling, negligible hyperfine interaction, and high electron mobility are obvious advantages for transporting spin information over long distances. However, such outstanding transport properties also limit the capability to engineer active spintronics, where strong spin-orbit coupling is crucial for creating and manipulating spin currents. To this end, transition metal dichalcogenides, which have larger spin-orbit coupling and good interface matching, appear to be highly complementary materials for enhancing the spin-dependent features of graphene while maintaining its superior charge transport properties. In this review, we present the theoretical framework and the experiments performed to detect and characterize the spin-orbit coupling and spin currents in graphene/transition metal dichalcogenide heterostructures. Specifically, we will concentrate on recent measurements of Hanle precession, weak antilocalization and the spin Hall effect, and provide a comprehensive theoretical description of the interconnection between these phenomena. © 2018 The Royal Society of Chemistry.


2017

  • Giant Spin Lifetime Anisotropy in Graphene Induced by Proximity Effects

    Cummings A.W., Garcia J.H., Fabian J., Roche S. Physical Review Letters; 119 (20, 206601) 2017. 10.1103/PhysRevLett.119.206601. IF: 8.462

    We report on fundamental aspects of spin dynamics in heterostructures of graphene and transition metal dichalcogenides (TMDCs). By using realistic models derived from first principles we compute the spin lifetime anisotropy, defined as the ratio of lifetimes for spins pointing out of the graphene plane to those pointing in the plane. We find that the anisotropy can reach values of tens to hundreds, which is unprecedented for typical 2D systems with spin-orbit coupling and indicates a qualitatively new regime of spin relaxation. This behavior is mediated by spin-valley locking, which is strongly imprinted onto graphene by TMDCs. Our results indicate that this giant spin lifetime anisotropy can serve as an experimental signature of materials with strong spin-valley locking, including graphene-TMDC heterostructures and TMDCs themselves. Additionally, materials with giant spin lifetime anisotropy can provide an exciting platform for manipulating the valley and spin degrees of freedom, and for designing novel spintronic devices. © 2017 American Physical Society.


  • Spin hall effect and weak antilocalization in graphene/transition metal dichalcogenide heterostructures

    Garcia J.H., Cummings A.W., Roche S. Nano Letters; 17 (8): 5078 - 5083. 2017. 10.1021/acs.nanolett.7b02364. IF: 12.712

    We report on a theoretical study of the spin Hall Effect (SHE) and weak antilocalization (WAL) in graphene/transition metal dichalcogenide (TMDC) heterostructures, computed through efficient real-space quantum transport methods, and using realistic tight-binding models parametrized from ab initio calculations. The graphene/WS2 system is found to maximize spin proximity effects compared to graphene on MoS2, WSe2, or MoSe2 with a crucial role played by disorder, given the disappearance of SHE signals in the presence of strong intervalley scattering. Notably, we found that stronger WAL effects are concomitant with weaker charge-to-spin conversion efficiency. For further experimental studies of graphene/TMDC heterostructures, our findings provide guidelines for reaching the upper limit of spin current formation and for fully harvesting the potential of two-dimensional materials for spintronic applications. © 2017 American Chemical Society.


  • Valley-polarized quantum transport generated by gauge fields in graphene

    Settnes M., Garcia J.H., Roche S. 2D Materials; 4 (3, 031006) 2017. 10.1088/2053-1583/aa7cbd. IF: 6.937

    We report on the possibility to simultaneously generate in graphene a bulk valley-polarized dissipative transport and a quantum valley Hall effect by combining strain-induced gauge fields and real magnetic fields. Such unique phenomenon results from a ‘resonance/anti-resonance’ effect driven by the superposition/cancellation of superimposed gauge fields which differently affect time reversal symmetry. The onset of a valley-polarized Hall current concomitant to a dissipative valley-polarized current flow in the opposite valley is revealed by a e2 /h Hall conductivity plateau. We employ efficient linear scaling Kubo transport methods combined with a valley projection scheme to access valley-dependent conductivities and show that the results are robust against disorder.


2016

  • Charge, spin and valley Hall effects in disordered grapheme

    Cresti A., Nikolíc B.K., Garćia J.H., Roche S. Rivista del Nuovo Cimento; 39 (12): 587 - 667. 2016. 10.1393/ncr/i2016-10130-6. IF: 1.250

    The discovery of the integer quantum Hall effect in the early eighties of the last century, with highly precise quantization values for the Hall conductance in multiples of e2/h, has been the first fascinating manifestation of the topological state of matter driven by magnetic field and disorder, and related to the formation of non-dissipative current flow. Throughout the 2000's, several new phenomena such as the spin Hall effect and the quantum spin Hall effect were confirmed experimentally for systems with strong spin-orbit coupling effects and in the absence of external magnetic field. More recently, the Zeeman spin Hall effect and the formation of valley Hall topological currents have been introduced for graphene-based systems, under time-reversal or inversion symmetry-breaking conditions, respectively. This review presents a comprehensive coverage of all these Hall effects in disordered graphene from the perspective of numerical simulations of quantum transport in two-dimensional bulk systems (by means of the Kubo formalism) and multiterminal nanostructures (by means of the Landauer-Buttiker scattering and non-equilibrium Green's function approaches). In contrast to usual two-dimensional electron gases in semiconductor heterostructures, the presence of defects in graphene generates more complex electronic features such as electron-hole asymmetry, defect-induced resonances in the electron density of states or percolation effect between localized impurity states, which, together with extra degrees of freedom (sublattice pseudospin and valley isospin), bring a higher degree of complexity and enlarge the transport phase diagram.