21 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
Crystal-field effects in graphene with interface-induced spin-orbit coupling
We consider theoretically the influence of crystalline fields on the electronic structure of graphene placed on a layered material with reduced symmetry and large spin-orbit coupling (SOC). We use a perturbative procedure combined with the Slater-Koster method to derive the low-energy effective Hamiltonian around the points and estimate the magnitude of the effective couplings. Two simple models for the envisaged graphene-substrate hybrid bilayer are considered, in which the relevant atomic orbitals hybridize with either top or hollow sites of the graphene honeycomb lattice. In both cases, the interlayer coupling to a crystal-field-split substrate is found to generate highly anisotropic proximity spin-orbit interactions, including in-plane 'spin-valley' coupling. Interestingly, when an anisotropic intrinsic-type SOC becomes sizeable, the bilayer system is effectively a quantum spin Hall insulator characterized by in-plane helical edge states robust against Bychkov-Rashba effect. Finally, we discuss the type of substrate required to achieve anisotropic proximity-induced SOC and suggest possible candidates to further explore crystal field effects in graphene-based heterostructures
The impact of Rashba spin-orbit coupling in charge-ordered systems
We study the impact of the Rashba spin-orbit coupling (RSOC) on the stability
of charge-density wave (CDW) in systems with large electron-phonon coupling
(EPC). Here, the EPC is considered in the framework of the Holstein model at
the half-filled square lattice. We start obtaining the phase diagram of the
Rashba-Holstein model using the Hartree-Fock mean-field theory, and identifying
the boundaries of the CDW and Rashba metal phases. As our main result, we
notice that the RSOC disfavors the CDW phase, driving the system to a
correlated Rashba metal. Proceeding, we employ a cluster perturbation theory
(CPT) approach to investigate the phase diagram beyond the Hartree-Fock
approximation. The quantum correlations captured by CPT indicate that the RSOC
is even more detrimental to CDW than previously anticipated. That is, the
Rashba metal region is observed to be expanded in comparison to the mean-field
case. Additionally, we investigate pairing correlations, and the results
further strengthen the identification of critical points.Comment: 9 pages, 8 figure
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
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
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
Controlling electric and magnetic Purcell effects in phosphorene via strain engineering
We investigate the spontaneous emission lifetime of a quantum emitter near a
substrate coated with phosphorene under the influence of uniaxial strain. We
consider both electric dipole and magnetic dipole-mediated spontaneous
transitions from the excited to the ground state. The modeling of phosphorene
is performed by employing a tight-binding model that goes beyond the usual
low-energy description. We demonstrate that both electric and magnetic decay
rates can be strongly tuned by the application of uniform strain, ranging from
a near-total suppression of the Purcell effect to a remarkable enhancement of
more than 1300% due to the high flexibility associated with the puckered
lattice structure of phosphorene. We also unveil the use of strain as a
mechanism to tailor the most probable decay pathways of the emitted quanta. Our
results show that uniaxially strained phosphorene is an efficient and versatile
material platform for the active control of light-matter interactions thanks to
its extraordinary optomechanical properties