128 research outputs found
Beating of the oscillations in the transport coefficients of a one-dimensionally periodically modulated two-dimensional electron gas in the presence of spin-orbit interaction
Transport properties of a two-dimensional electron gas (2DEG) are studied in
the presence of a perpendicular magnetic field , of a {\it weak}
one-dimensional (1D) periodic potential modulation, and of the spin-orbit
interaction (SOI) described only by the Rashba term. In the absence of the
modulation the SOI mixes the spin-up and spin-down states of neighboring Landau
levels into two new, unequally spaced energy branches. The levels of these
branches broaden into bands in the presence of the modulation and their
bandwidths oscillate with the field . Evaluated at the Fermi energy, the
-th level bandwidth of each series has a minimum or vanishes at different
values of the field . In contrast with the 1D-modulated 2DEG without SOI,
for which only one flat-band condition applies, here there are two flat-band
conditions that can change considerably as a function of the SOI strength
and accordingly influence the transport coefficients of the 2DEG. The
phase and amplitude of the Weiss and Shubnikov-de Haas (SdH) oscillations
depend on the strength . For small values of both oscillations
show beating patterns. Those of the former are due to the independently
oscillating bandwidths whereas those of the latter are due to modifications of
the density of states, exhibit an even-odd filling factor transition, and are
nearly independent of the modulation strength. For strong values of
the SdH oscillations are split in two
Visibility diagrams and experimental stripe structure in the quantum Hall effect
We analyze various properties of the visibility diagrams that can be used in
the context of modular symmetries and confront them to some recent experimental
developments in the Quantum Hall Effect. We show that a suitable physical
interpretation of the visibility diagrams which permits one to describe
successfully the observed architecture of the Quantum Hall states gives rise
naturally to a stripe structure reproducing some of the experimental features
that have been observed in the study of the quantum fluctuations of the Hall
conductance. Furthermore, we exhibit new properties of the visibility diagrams
stemming from the structure of subgroups of the full modular group.Comment: 8 pages in plain TeX, 7 figures in a single postscript fil
Magnetic Miniband Structure and Quantum Oscillations in Lateral Semiconductor Superlattices
We present fully quantum-mechanical magnetotransport calculations for
short-period lateral superlattices with one-dimensional electrostatic
modulation. A non-perturbative treatment of both magnetic field and modulation
potential proves to be necessary to reproduce novel quantum oscillations in the
magnetoresistance found in recent experiments in the resistance component
parallel to the modulation potential. In addition, we predict oscillations of
opposite phase in the component perpendicular to the modulation not yet
observed experimentally. We show that the new oscillations originate from the
magnetic miniband structure in the regime of overlapping minibands.Comment: 6 pages with 4 figure
Singular Effects of Spin-Flip Scattering on Gapped Dirac Fermions
We investigate the effects of spin-flip scattering on the Hall transport and
spectral properties of gapped Dirac fermions. We find that in the weak
scattering regime, the Berry curvature distribution is dramatically compressed
in the electronic energy spectrum, becoming singular at band edges. As a result
the Hall conductivity has a sudden jump (or drop) of when the Fermi
energy sweeps across the band edges, and otherwise is a constant quantized in
units of . In parallel, spectral properties such as the density of
states and spin polarization are also greatly enhanced at band edges. Possible
experimental methods to detect these effects are discussed
Two-dimensional electron gas in a uniform magnetic field in the presence of a delta-impurity
The density of states and the Hall conductivity of a two-dimensional electron
gas in a uniform magnetic field and in the presence of a delta impurity are
exactly calculated using elementary field theoretic techniques. Although these
results are not new, our treatment is explicitly gauge-invariant, and can be
easily adapted to other problems involving a delta potential.Comment: 12+1 pages, 1 ps figure, REVTEX. Corrigendum adde
Electric-Field Breakdown of Absolute Negative Conductivity and Supersonic Streams in Two-Dimensional Electron Systems with Zero Resistance/Conductance States
We calculate the current-voltage characteristic of a two-dimensional electron
system (2DES) subjected to a magnetic field at strong electric fields. The
interaction of electrons with piezoelectric acoustic phonons is considered as a
major scattering mechanism governing the current-voltage characteristic. It is
shown that at a sufficiently strong electric field corresponding to the Hall
drift velocity exceeding the velocity of sound, the dissipative current
exhibits an overshoot. The overshoot of the dissipative current can result in a
breakdown of the absolute negative conductivity caused by microwave irradiation
and, therefore, substantially effect the formation of the domain structures
with the zero-resistance and zero-conductance states and supersonic electron
streams.Comment: 5 pages, 4 figure
Anomalous Hall effect in 2D Dirac band: link between Kubo-Streda formula and semiclassical Boltzmann equation approach
The anomalous Hall effect (AHE) is a consequence of spin-orbit coupling in a
ferromagnetic metal and is related primarily to density-matrix response to an
electric field that is off-diagonal in band index. For this reason disorder
contributions to the AHE are difficult to treat systematically using a
semi-classical Boltzmann equation approach, even when weak localization
corrections are disregarded. In this article we explicitly demonstrate the
equivalence of an appropriately modified semiclassical transport theory which
includes anomalous velocity and side jump contributions and microscopic
Kubo-Streda perturbation theory, with particular unconventional contributions
in the semiclassical theory identified with particular Feynman diagrams when
calculations are carried out in a band-eigenstate representation. The
equivalence we establish is verified by explcit calculations for the case of
the two-dimensional (2D) Dirac model Hamiltonian relevant to graphene.Comment: 17 pages, 13 figure
Weak Localization and Integer Quantum Hall Effect in a Periodic Potential
We consider magnetotransport in a disordered two-dimensional electron gas in
the presence of a periodic modulation in one direction. Existing quasiclassical
and quantum approaches to this problem account for Weiss oscillations in the
resistivity tensor at moderate magnetic fields, as well as a strong
modulation-induced modification of the Shubnikov-de Haas oscillations at higher
magnetic fields. They do not account, however, for the operation at even higher
magnetic fields of the integer quantum Hall effect, for which quantum
interference processes are responsible. We then introduce a field-theory
approach, based on a nonlinear sigma model, which encompasses naturally both
the quasiclassical and quantum-mechanical approaches, as well as providing a
consistent means of extending them to include quantum interference corrections.
A perturbative renormalization-group analysis of the field theory shows how
weak localization corrections to the conductivity tensor may be described by a
modification of the usual one-parameter scaling, such as to accommodate the
anisotropy of the bare conductivity tensor. We also show how the two-parameter
scaling, conjectured as a model for the quantum Hall effect in unmodulated
systems, may be generalized similarly for the modulated system. Within this
model we illustrate the operation of the quantum Hall effect in modulated
systems for parameters that are realistic for current experiments.Comment: 15 pages, 4 figures, ReVTeX; revised version with condensed
introduction; two figures taken out; reference adde
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