71 research outputs found
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
Ultrathin films of black phosphorus as suitable platforms for unambiguous observation of the orbital Hall effect
Phosphorene, a monolayer of black phosphorus, is a two-dimensional material
that lacks a multivalley structure in the Brillouin zone and has negligible
spin-orbit coupling. This makes it a promising candidate for investigating the
orbital Hall effect independently of the valley or spin Hall effects. To model
phosphorene, we utilized a DFT-derived tight-binding Hamiltonian, which is
constructed with the pseudo atomic orbital projection method. For that purpose,
we use the PAOFLOW code with a newly implemented internal basis that provides a
fairly good description of the phosphorene conduction bands. By employing
linear response theory, we show that phosphorene exhibits a sizable orbital
Hall effect with strong anisotropy in the orbital Hall conductivity for the
out-of-plane orbital angular momentum component. The magnitude and sign of the
conductivity depend upon the in-plane direction of the applied electric field.
These distinctive features enable the observation of the orbital Hall effect in
this material unambiguously. The effects of strain and of a perpendicularly
applied electric field on the phosphorene orbital-Hall response are also
explored. We show that a supplementary electric field applied perpendicular to
the phosphorene layer in its conductive regime gives rise to an induced
in-plane orbital magnetization.Comment: 8 pages, 4 figure
Disentangling orbital and valley Hall effects in bilayers of transition metal dichalcogenides
It has been recently shown that monolayers of transition metal
dichalcogenides (TMDs) in the 2H structural phase exhibit relatively large
orbital Hall conductivity plateaus within their energy band gaps, where their
spin Hall conductivities vanish. However, since the valley Hall effect (VHE) in
these systems also generates a transverse flow of orbital angular momentum it
becomes experimentally challenging to distinguish between the two effects in
these materials. The VHE requires inversion symmetry breaking to occur, which
takes place in the TMD monolayers, but not in the bilayers. We show that a
bilayer of 2H-MoS is an orbital Hall insulator that exhibits a sizeable OHE
in the absence of both spin and valley Hall effects. This phase can be
characterised by an orbital Chern number that assumes the value
for the 2H-MoS bilayer and for the
monolayer, confirming the topological nature of these orbital-Hall insulator
systems. Our results are based on density functional theory (DFT) and
low-energy effective model calculations and strongly suggest that bilayers of
TMDs are highly suitable platforms for direct observation of the orbital Hall
insulating phase in two-dimensional materials. Implications of our findings for
attempts to observe the VHE in TMD bilayers are also discussed.Comment: 7 pages, 4 figures + Supplementary materia
Orbital magnetoelectric effect in zigzag nanoribbons of p-band systems
Profiles of the spin and orbital angular momentum accumulations induced by a
longitudinally applied electric field are explored in nanoribbons of -band
systems with a honeycomb lattice. We show that nanoribbons with zigzag borders
can exhibit orbital magnetoelectric effects. More specifically, we have found
that purely orbital magnetization oriented perpendicularly to the ribbon may be
induced in these systems by means of the external electric field, when
sublattice symmetry is broken. The effect is rather general and may occur in
other multi-orbital materials.Comment: 10 pages, 4 figure
Clone Embrapa 51: uma alternativa para resistênica à resinose-do-cajueiro.
São apresentados os resultados de cinco anos de monitoramento de clones comerciais de cajueiro, quanto à reação de resinose.bitstream/CNPAT/10571/1/cot_130.pd
Orbital magnetoelectric effect in nanoribbons of transition metal dichalcogenides
The orbital magnetoelectric effect (OME) generically refers to the appearance
of an orbital magnetization induced by an applied electric field. Here, we show
that nanoribbons of transition metal dichalcogenides (TMDs) with zigzag (ZZ)
edges may exhibit a sizeable OME activated by an electric field applied along
the ribbons' axis. We examine nanoribbons extracted from a monolayer (1L) and a
bilayer (2L) of MoS in the trigonal (H) structural phase. Transverse
profiles of the induced orbital angular momentum accumulations are calculated
to first order in the longitudinally applied electric field. Our results show
that close to the nanoribbon's edge-state crossings energy, the orbital angular
momentum accumulations take place mainly around the ribbons' edges. They have
two contributions: one arising from the orbital Hall effect (OHE) and the other
consists in the OME. The former is transversely anti-symmetric with respect to
the principal axis of the nanoribbon, whereas the latter is symmetric, and
hence responsible for the resultant orbital magnetization induced in the
system. We found that the orbital accumulation originating from the OHE for the
1L-nanoribbon is approximately half that of a 2L-nanoribbon. Furthermore, while
the OME can reach fairly high values in 1L-TMD nanoribbons, it vanishes in the
2L ones that preserve spatial inversion symmetry. The microscopic features that
justify our findings are also discussed.Comment: 14 pages, 16 figure
Transmissão de Lasiodiplodia theobromae, agente da resinose, em propágulos de cajueiro.
bitstream/CNPAT-2010/11983/1/Bd-034.pd
Numerical calculation of the Casimir-Polder interaction between a graphene sheet with vacancies and an atom
In this work the Casimir-Polder interaction energy between a rubidium atom and a disordered graphene sheet is investigated beyond the Dirac cone approximation by means of accurate real-space tight-binding calculations. As a model of defected graphene, we consider a tight-binding model of π electrons on a honeycomb lattice with a small concentration of vacancies. The optical response of the graphene sheet is evaluated with full spectral resolution by means of exact Chebyshev polynomial expansions of the Kubo formula in large lattices in excess of 10 million atoms. At low temperatures, the optical response of defected graphene is found to display two qualitatively distinct behaviors with a clear transition around finite (nonzero) Fermi energy. In the vicinity of the Dirac point, the imaginary part of optical conductivity is negative for low frequencies while the real part is strongly suppressed. On the other hand, for high doping, it has the same features found in the Drude model within the Dirac cone approximation, namely, a Drude peak at small frequencies and a change of sign in the imaginary part above the interband threshold. These characteristics translate into a nonmonotonic behavior of the Casimir-Polder interaction energy with very small variation with doping in the vicinity of the neutrality point while having the same form of the interaction calculated with Drude's model at high electronic density
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