318 research outputs found
Spin-charge conversion in disordered two-dimensional electron gases lacking inversion symmetry
We study the spin-charge conversion mechanisms in a two-dimensional gas of
electrons moving in a smooth disorder potential by accounting for both
Rashba-type and Mott's skew scattering contributions. We find that quantum
interference effects between spin-flip and skew scattering give rise to
anisotropic spin precession scattering (ASP), a direct spin-charge conversion
mechanism that was discovered in an earlier study of graphene decorated with
adatoms [C. Huang \emph{et al.} Phys.~Rev.~B \textbf{94} 085414.~(2016)]. Our
findings suggest that, together with other spin-charge conversion mechanisms
such as the inverse galvanic effect, ASP is a fairly universal phenomenon that
should be present in disordered two-dimensional systems lacking inversion
symmetry.Comment: 9 pages, 2 figure
Anomalous Nonlocal Resistance and Spin-charge Conversion Mechanisms in Two-Dimensional Metals
We uncover two anomalous features in the nonlocal transport behavior of
two-dimensional metallic materials with spin-orbit coupling. Firstly, the
nonlocal resistance can have negative values and oscillate with distance, even
in the absence of a magnetic field. Secondly, the oscillations of the nonlocal
resistance under an applied in-plane magnetic field (Hanle effect) can be
asymmetric under field reversal. Both features are produced by direct
magnetoelectric coupling, which is possible in materials with broken inversion
symmetry but was not included in previous spin diffusion theories of nonlocal
transport. These effects can be used to identify the relative contributions of
different spin-charge conversion mechanisms. They should be observable in
adatom-functionalized graphene, and may provide the reason for discrepancies in
recent nonlocal transport experiments on graphene.Comment: 5 pages, 3 figures, and Supplementary Materials, to appear in Phys.
Rev. Let
Control of Spin Diffusion and Suppression of the Hanle Effect by the Coexistence of Spin and Valley Hall Effects
In addition to spin, electrons in many materials possess an additional
pseudo-spin degree of freedom known as 'valley'. In materials where the spin
and valley degrees of freedom are weakly coupled, they can be both excited and
controlled independently. In this work, we study a model describing the
interplay of the spin and valley Hall effects in such two-dimensional
materials. We demonstrate the emergence of an additional longitudinal neutral
current that is both spin and valley polarized. The additional neutral current
allows to control the spin density by tuning the magnitude of the valley Hall
effect. In addition, the interplay of the two effects can suppress the Hanle
effect, that is, the oscillation of the nonlocal resistance of a Hall bar
device with in-plane magnetic field. The latter observation provides a possible
explanation for the absence of the Hanle effect in a number of recent
experiments. Our work opens also the possibility to engineer the conversion
between the valley and spin degrees of freedom in two-dimensional materials.Comment: 15 pages, 2 figure
Graphene Electrodynamics in the presence of the Extrinsic Spin Hall Effect
We extend the electrodynamics of two dimensional electron gases to account
for the extrinsic spin Hall effect (SHE). The theory is applied to doped
graphene decorated with a random distribution of absorbates that induce
spin-orbit coupling (SOC) by proximity. The formalism extends previous
semiclassical treatments of the SHE to the non-local dynamical regime. Within a
particle-number conserving approximation, we compute the conductivity,
dielectric function, and spin Hall angle in the small frequency and wave vector
limit. The spin Hall angle is found to decrease with frequency and wave number,
but it remains comparable to its zero-frequency value around the frequency
corresponding to the Drude peak. The plasmon dispersion and linewidth are also
obtained. The extrinsic SHE affects the plasmon dispersion in the long
wavelength limit, but not at large values of the wave number. This result
suggests an explanation for the rather similar plasmonic response measured in
exfoliated graphene, which does not exhibit the SHE, and graphene grown by
chemical vapor deposition, for which a large SHE has been recently reported.
Our theory also lays the foundation for future experimental searches of SOC
effects in the electrodynamic response of two-dimensional electron gases with
SOC disorder.Comment: 12 pages, 4 figure
Unconventional Metallic Magnetism: Non-analyticity and Sign-changing Behavior of Orbital Magnetization in ABC Trilayer Graphene
We study an unique form of metallic ferromagnetism in which orbital moments
surpasses the role of spin moments in shaping the overall magnetization. This
system emerges naturally upon doping a topologically non-trivial Chern band in
the recently identified quarter metal phase of rhombohedral trilayer graphene.
Our comprehensive scan of the density-interlayer potential parameter space
reveals an unexpected landscape of orbital magnetization marked by two sign
changes and a line of singularities. The sign change originates from an intense
Berry curvature concentrated close to the band-edge, and the singularity arises
from a topological Lifshitz transition that transform a simply connected Fermi
sea into an annular Fermi sea. Importantly, these variations occur while the
groundstate order-parameter (i.e.~valley and spin polarization) remains
unchanged. This unconventional relationship between the order parameter and
magnetization markedly contrasts traditional spin ferromagnets, where spin
magnetization is simply proportional to the groundstate spin polarization via
the gyromagnetic ratio. We compute energy and magnetization curves as functions
of collective valley rotation to shed light on magnetization dynamics and to
expand the Stoner-Wohlfarth magnetization reversal model. We provide
predictions on the magnetic coercive field that can be readily tested in
experiments. Our results challenge established perceptions of magnetism,
emphasising the important role of orbital moments in two-dimensional materials
such as graphene and transition metal dichalcogenides, and in turn, expand our
understanding and potential manipulation of magnetic behaviors in these
systems.Comment: 4+8 page
Edge Modes, Degeneracies, and Topological Numbers in Non-Hermitian Systems
We analyze chiral topological edge modes in a non-Hermitian variant of the 2D
Dirac equation. Such modes appear at interfaces between media with different
"masses" and/or signs of the "non-Hermitian charge". The existence of these
edge modes is intimately related to exceptional points of the bulk
Hamiltonians, i.e., degeneracies in the bulk spectra of the media. We find that
the topological edge modes can be divided into three families
("Hermitian-like", "non-Hermitian", and "mixed"), these are characterized by
two winding numbers, describing two distinct kinds of half-integer charges
carried by the exceptional points. We show that all the above types of
topological edge modes can be realized in honeycomb lattices of ring resonators
with asymmetric or gain/loss couplings.Comment: 6 pages, 3 figures, and Supplementary Materials, to appear in Phys.
Rev. Let
Quasi-boson approximation yields accurate correlation energy in the 2D electron gas
We report the successful adaptation of the quasi-boson approximation, a
technique traditionally employed in nuclear physics, to the analysis of the
two-dimensional electron gas. We show that the correlation energy estimated
from this approximation agrees closely with the results obtained from quantum
Monte Carlo simulations. Our methodology comprehensively incorporates the
exchange self-energy, direct scattering, and exchange scattering for a
particle-hole pair excited out of the mean-field groundstate within the
equation-of-motion framework. The linearization of the equation of motion leads
to a generalized-random-phase-approximation (gRPA) eigenvalue equation whose
spectrum indicates that the plasmon dispersion remains unaffected by exchange
effects, while the particle-hole continuum experiences a marked upward shift
due to the exchange self-energy. Notably, the plasmon mode retains its
collective nature within the particle-hole continuum, up to moderately short
wavelength ( at metallic density ). Using the gRPA
excitation spectrum, we calculate the zero-point energy of the quasi-boson
Hamiltonian, thereby approximating the correlation energy of the original
Hamiltonian. This research highlights the potential and effectiveness of
applying the quasi-boson approximation to the gRPA spectrum, a fundamental
technique in nuclear physics, to extended condensed matter systems.Comment: 7 pages, 4 figure
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