4,735 research outputs found
Spin Hall effect and Weak Antilocalization in Graphene/Transition Metal Dichalcogenide Heterostructures
We report on a theoretical study of the spin Hall Effect (SHE) and weak
antilocal-ization (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/WS 2 system is found to maximize spin proximity
effects compared to graphene on MoS 2 , WSe 2 , or MoSe 2 , 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.Comment: This document is the unedited Author's version of a Submitted Work
that was subsequently accepted for publication in Nano Letters,
copyright\c{opyright}American Chemical Society after peer review. To access
the final edited and published work see
http://pubs.acs.org/articlesonrequest/AOR-c2pZ8WnmG7pcF4MIivj
Spin transport in graphene/transition metal dichalcogenide heterostructures
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.Comment: 21 pages, 11 figures. This document is the unedited Author's version
of a Submitted Work that was subsequently accepted for publication in Nano
Letters, copyright\c{opyright}American Chemical Society after peer review. To
access the final edited and published work see
http://pubs.rsc.org/en/Content/ArticleLanding/2018/CS/C7CS00864
Adatoms and Anderson localization in graphene
We address the nature of the disordered state that results from the
adsorption of adatoms in graphene. For adatoms that sit at the center of the
honeycomb plaquette, as in the case of most transition metals, we show that the
ones that form a zero-energy resonant state lead to Anderson localization in
the vicinity of the Dirac point. Among those, we show that there is a symmetry
class of adatoms where Anderson localization is suppressed, leading to an
exotic metallic state with large and rare charge droplets, that localizes only
at the Dirac point. We identify the experimental conditions for the observation
of the Anderson transition for adatoms in graphene.Comment: 8 pages, 5 figures, 2 appendixes, Final versio
Quantum Hall Effect in Graphene with Interface-Induced Spin-Orbit Coupling
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 analysing 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 characterise
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
Spin Filtering and Entanglement Swapping through Coherent Evolution of a Single Quantum Dot
We exploit the non-dissipative dynamics of a pair of electrons in a large
square quantum dot to perform singlet-triplet spin measurement through a single
charge detection and show how this may be used for entanglement swapping and
teleportation. The method is also used to generate the AKLT ground state, a
further resource for quantum computation. We justify, and derive analytic
results for, an effective charge-spin Hamiltonian which is valid over a wide
range of parameters and agrees well with exact numerical results of a realistic
effective-mass model. Our analysis also indicates that the method is robust to
choice of dot-size and initialization errors, as well as decoherence introduced
by the hyperfine interaction.Comment: 5 pages, 3 figure
Valley-Polarized Quantum Anomalous Hall Phase in Bilayer Graphene with Layer-Dependent Proximity Effects
Realizations of some topological phases in two-dimensional systems rely on
the challenge of jointly incorporating spin-orbit and magnetic exchange
interactions. Here, we predict the formation and control of a fully
valley-polarized quantum anomalous Hall effect in bilayer graphene, by
separately imprinting spin-orbit and magnetic proximity effects in different
layers. This results in varying spin splittings for the conduction and valence
bands, which gives rise to a topological gap at a single Dirac cone. The
topological phase can be controlled by a gate voltage and switched between
valleys by reversing the sign of the exchange interaction. By performing
quantum transport calculations in disordered systems, the chirality and
resilience of the valley-polarized edge state are demonstrated. Our findings
provide a promising route to engineer a topological phase that could enable
low-power electronic devices and valleytronic applications.Comment: Final published version. PRB letter:
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.104.L16111
Real-space calculation of the conductivity tensor for disordered topological matter
We describe an efficient numerical approach to calculate the longitudinal and
transverse Kubo conductivities of large systems using Bastin's formulation. We
expand the Green's functions in terms of Chebyshev polynomials and compute the
conductivity tensor for any temperature and chemical potential in a single
step. To illustrate the power and generality of the approach, we calculate the
conductivity tensor for the quantum Hall effect in disordered graphene and
analyze the effect of the disorder in a Chern insulator in Haldane's model on a
honeycomb lattice.Comment: 5 pages, 3 figures and a supplementary material (3 pages
Kinetic Modeling of Vacuum Gas Oil Hydrotreatment using a Molecular Reconstruction Approach
International audienceVacuum Gas Oils (VGO) are heavy petroleum cuts (boiling points ranging from 350 to 550 ËšC) that can be transformed into valuable fuels (gasolines, diesels) by fluid catalytic cracking or hydrocracking. Prior to these conversion processes, hydrotreating is required in order to eliminate the impurities in VGOs. The hydrotreatment process enables to meet the environmental specifications (total sulfur contents) and to prevent nitrogen poisoning of conversion catalysts. In order to develop a kinetic model based on an accurate VGOs molecular description, innovative analytical tools and molecular reconstruction techniques were used in this work. A lumped model using a Langmuir-Hinshelwood representation was developed for hydrodearomatization, hydrodesulfurization and hydrodenitrogenation of the VGO. This lumped model was successfully applied to the experimental feed pretreatment data and was able to predict evolution of concentration of the aromatics, nitrogen and sulfur species
Magnetism, symmetry and spin transport in van der Waals layered systems
The discovery of an ever-increasing family of atomic layered magnetic materials, together with the already established vast catalogue of strong spin–orbit coupling and topological systems, calls for some guiding principles to tailor and optimize novel spin transport and optical properties at their interfaces. Here, we focus on the latest developments in both fields that have brought them closer together and make them ripe for future fruitful synergy. After outlining fundamentals on van der Waals magnetism and spin–orbit coupling effects, we discuss how their coexistence, manipulation and competition could ultimately establish new ways to engineer robust spin textures and drive the generation and dynamics of spin current and magnetization switching in 2D-materials-based van der Waals heterostructures. Grounding our analysis on existing experimental results and theoretical considerations, we draw a prospective analysis about how intertwined magnetism and spin–orbit torque phenomena combine at interfaces with well-defined symmetries and how this dictates the nature and figures of merit of spin–orbit torque and angular momentum transfer. This will serve as a guiding role in designing future non-volatile memory devices that utilize the unique properties of 2D materials with the spin degree of freedom
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