28 research outputs found
Pseudo-electromagnetic fields in topological semimetals
Dirac and Weyl semimetals, materials where electrons behave as relativistic
fermions, react to position- and time-dependent perturbations, such as strain,
as if emergent electromagnetic fields were applied. Since they differ from
external electromagnetic fields in their symmetries and phenomenology they are
called pseudo-electromagnetic fields, and enable a simple and unified
description of a variety of inhomogeneous systems involving topological
semimetals. We review the different physical ways to create effective
pseudo-fields, their observable consequences as well as their similarities and
differences compared to electromagnetic fields. Among these difference is their
effect on quantum anomalies, the absence of a classical symmetry in the quantum
theory, which we revisit from a quantum field theory and a semiclassical
viewpoint. We conclude with predicted observable signatures of the
pseudo-fields and the nascent experimental status.Comment: 18 pages, 6 (preliminary) figures. Original submitted version,
comments welcom
Robust Helical Edge Transport in Quantum Spin Hall Quantum Wells
We show that burying of the Dirac point in semiconductor-based
quantum-spin-Hall systems can generate unexpected robustness of edge states to
magnetic fields. A detailed band-structure analysis reveals
that InAs/GaSb and HgTe/CdTe quantum wells exhibit such buried Dirac points. By
simulating transport in a disordered system described within an effective
model, we further demonstrate that buried Dirac points yield nearly quantized
edge conduction out to large magnetic fields, consistent with recent
experiments.Comment: 11 pages, 6 figure
Noise-induced backscattering in a quantum-spin-Hall edge
Time-reversal symmetry suppresses electron backscattering in a
quantum-spin-Hall edge, yielding quantized conductance at zero temperature.
Understanding the dominant corrections in finite-temperature experiments
remains an unsettled issue. We study a novel mechanism for conductance
suppression: backscattering caused by incoherent electromagnetic noise.
Specifically, we show that an electric potential fluctuating randomly in time
can backscatter electrons inelastically without constraints faced by
electron-electron interactions. We quantify noise-induced corrections to the dc
conductance in various regimes and propose an experiment to test this scenario.Comment: 5+3 pages, 1 figure; shortened version to appear in PRL, added
reference
Quantum-metric-enabled exciton condensate in double twisted bilayer graphene
Flat-band systems are a promising platform for realizing exotic collective
ground states with spontaneously broken symmetry because the electron-electron
interactions dominate the kinetic energy. A state of particular interest would
be the chased after interlayer-phase-coherent exciton condensate but the
conventional treatments of the effect of thermal and quantum fluctuations
suggest that this state might be either unstable or fragile. In this work,
using double twisted bilayer graphene heterostructures as an example, we show
that the quantum metric of the Bloch wave functions can strongly stabilize the
exciton condensate and reverse the conclusion that one would draw using a
conventional approach. The quantum metric contribution arises from interband
terms, and flat-bands are most commonly realized by engineering multiband
systems. Our results therefore suggest that in many practical situations the
quantum metric can play a critical role in determining the stability of exciton
condensates in double layers formed by two-dimensional systems with flat-bands.Comment: 6 pages, 6 figure
Suppressing quasiparticle poisoning with a voltage-controlled filter
We study single-electron charging events in an Al/InAs nanowire hybrid system
with deliberately introduced gapless regions. The occupancy of a Coulomb island
is detected using a nearby radio-frequency quantum dot as a charge sensor. We
demonstrate that a 1 micron gapped segment of the wire can be used to
efficiently suppress single electron poisoning of the gapless region and
therefore protect the parity of the island while maintaining good electrical
contact with a normal lead. In the absence of protection by charging energy,
the 1e switching rate can be reduced below 200 per second. In the same
configuration, we observe strong quantum charge fluctuations due to exchange of
electron pairs between the island and the lead. The magnetic field dependence
of the poisoning rate yields a zero-field superconducting coherence length of ~
90 nm