18 research outputs found
Two-dimensional topological insulator edge state backscattering by dephasing
To understand the seemingly absent temperature dependence in the conductance
of two-dimensional topological insulator edge states, we perform a numerical
study which identifies the quantitative influence of the combined effect of
dephasing and elastic scattering in charge puddles close to the edges. We show
that this mechanism may be responsible for the experimental signatures in
HgTe/CdTe quantum wells if the puddles in the samples are large and weakly
coupled to the sample edges. We propose experiments on artificial puddles which
allow to verify this hypothesis and to extract the real dephasing time scale
using our predictions. In addition, we present a new method to include the
effect of dephasing in wave-packet-time-evolution algorithms.Comment: 7 pages, 5 figure
Self-consistent calculation of electric potentials in Hall devices
Using a first-principles classical many-body simulation of a Hall bar, we
study the necessary conditions for the formation of the Hall potential: (i)
Ohmic contacts with metallic reservoirs, (ii) electron-electron interactions,
and (iii) confinement to a finite system. By propagating thousands of
interacting electrons over million time-steps we capture the build-up of the
self-consistent potential, which resembles results obtained by
conformal-mapping methods. As shown by a microscopic model of the current
injection, the Hall effect is linked to specific boundary conditions at the
particle reservoirs.Comment: 6 pages, 7 figure
Wave packet approach to transport in mesoscopic systems
Wave packets provide a well established and versatile tool for studying
time-dependent effects in molecular physics. Here, we demonstrate the
application of wave packets to mesoscopic nanodevices at low temperatures. The
electronic transport in the devices is expressed in terms of scattering and
transmission coefficients, which are efficiently obtained by solving an initial
value problem (IVP) using the time-dependent Schroedinger equation. The
formulation as an IVP makes non-trivial device topologies accessible and by
tuning the wave packet parameters one can extract the scattering properties for
a large range of energies.Comment: 12 pages, 4 figure
Weak localization in mesoscopic hole transport: Berry phases and classical correlations
We consider phase-coherent transport through ballistic and diffusive
two-dimensional hole systems based on the Kohn-Luttinger Hamiltonian. We show
that intrinsic heavy-hole light-hole coupling gives rise to clear-cut
signatures of an associated Berry phase in the weak localization which renders
the magneto-conductance profile distinctly different from electron transport.
Non-universal classical correlations determine the strength of these Berry
phase effects and the effective symmetry class, leading even to
antilocalization-type features for circular quantum dots and Aharonov-Bohm
rings in the absence of additional spin-orbit interaction. Our semiclassical
predictions are quantitatively confirmed by numerical transport calculations
Theory of the quantum Hall effect in graphene
We study the quantum Hall effect (QHE) in graphene based on the current
injection model. In our model, the presence of disorder, the edge-state
picture, extended states and localized states, which are believed to be
indispensable ingredients in describing the QHE, do not play an important role.
Instead the boundary conditions during the injection into the graphene sheet,
which are enforced by the presence of the Ohmic contacts, determine the
current-voltage characteristics.Comment: 4 pages, 3 figures, rewritten, role of contacts for boundary
conditions in small device
Using Topological Insulator Proximity to Generate Perfectly Conducting Channels in Materials without Topologie Protection
We show that hybrid structures of topological insulators and materials without topological protection can be employed to create perfectly conducting channels hosted in the non-topological part. These states inherit the topological protection from the proximity of the topological insulator but are more fragile to time-reversal symmetry breaking because of their extended character. We explore their formation in the band structure of model hybrid systems as well as realistic heterostructures involving HgTe/CdTe-based two-dimensional topological insulators. Using numerical quantum transport calculations for the HgTe/CdTe material system we propose two experimental settings which allow for the detection of the induced perfectly conducting channels, both in the localized and diffusive regime, by means of magneto conductance and shot noise
Revivals of quantum wave-packets in graphene
We investigate the propagation of wave-packets on graphene in a perpendicular
magnetic field and the appearance of collapses and revivals in the
time-evolution of an initially localised wave-packet. The wave-packet evolution
in graphene differs drastically from the one in an electron gas and shows a
rich revival structure similar to the dynamics of highly excited Rydberg
states.
We present a novel numerical wave-packet propagation scheme in order to solve
the effective single-particle Dirac-Hamiltonian of graphene and show how the
collapse and revival dynamics is affected by the presence of disorder. Our
effective numerical method is of general interest for the solution of the Dirac
equation in the presence of potentials and magnetic fields.Comment: 22 pages, 10 figures, 3 movies, to appear in New Journal of Physic
Fabry-P\'erot interference in gapped bilayer graphene with broken anti-Klein tunneling
We report the experimental observation of Fabry-P\'erot (FP) interference in
the conductance of a gate-defined cavity in a dual-gated bilayer graphene (BLG)
device. The high quality of the BLG flake, combined with the device's
electrical robustness provided by the encapsulation between two hexagonal boron
nitride layers, allows us to observe ballistic phase-coherent transport through
a {\mu}m-long cavity. We confirm the origin of the observed interference
pattern by comparing to tight-binding calculations accounting for the
gate-tunable bandgap. The good agreement between experiment and theory, free of
tuning parameters, further verifies that a gap opens in our device. The gap is
shown to destroy the perfect reflection for electrons traversing the barrier
with normal incidence (anti-Klein tunneling). The broken anti-Klein tunneling
implies that the Berry phase, which is found to vary with the gate voltages, is
always involved in the FP oscillations regardless of the magnetic field, in
sharp contrast with single-layer graphene.Comment: 5 pages, 4 figure
Towards a quantum time mirror for non-relativistic wave packets
We propose a method – a quantum time mirror (QTM) – for simulating a partial time-reversal of the free-space motion of a nonrelativistic quantum wave packet. The method is based on a short-time spatially-homogeneous perturbation to the wave packet dynamics, achieved by adding a nonlinear time-dependent term to the underlying Schroedinger equation. Numerical calculations, supporting our analytical considerations, demonstrate the effectiveness of the proposed QTM for generating a time-reversed echo image of initially localized matter-wave packets in one and two spatial dimensions. We also discuss possible experimental realizations of the proposed QTM