248 research outputs found
Signatures of topology in ballistic bulk transport of HgTe quantum wells
We calculate bulk transport properties of two-dimensional topological
insulators based on HgTe quantum wells in the ballistic regime. Interestingly,
we find that the conductance and the shot noise are distinctively different for
the so-called normal regime (the topologically trivial case) and the so-called
inverted regime (the topologically non-trivial case). Thus, it is possible to
verify the topological order of a two-dimensional topological insulator not
only via observable edge properties but also via observable bulk properties.
This is important because we show that under certain conditions the bulk
contribution can dominate the edge contribution which makes it essential to
fully understand the former for the interpretation of future experiments in
clean samples.Comment: 5 pages, 4 figure
Fraunhofer pattern in the presence of Majorana zero modes
Majorana bound states (MBSs) emerge as zero energy excitations in topological
superconductors. At zero temperature, their presence gives a quantized
conductance in NS junctions and a fractional Josephson effect in Josephson
junctions when the parity is conserved. However, most of current experiments
deviate from the theoretical predictions, yielding for example a non-quantized
conductance or the absence of only few odd Shapiro steps. Although these
results might be compatible with a topological ground state, it is also
possible that a trivial scenario can mimic similar results, by means of
accidental zero energy Andreev bound states (ZEABS) or simply by non-adiabatic
transitions between trivial Andreev bound states. Here, we propose a new
platform to investigate signatures of the presence of MBSs in the Fraunhofer
pattern of Josephson junctions featuring quantum spin Hall edge states on the
normal part and Majorana bound states at the NS interfaces. We use a
tight-binding model to demonstrate a change in periodicity of the Fraunhofer
pattern when comparing trivial and non-trivial regimes. We explain these
results in terms of local and crossed Andreev bound states, which due to the
spin-momentum locking, accumulate different magnetic flux and therefore become
distinguishable in the Fraunhofer periodicity. Furthermore, we introduce a
scattering model that captures the main results of the microscopic calculations
with MBSs and extend our discussion to the main differences found using
accidental ZEABS.Comment: 17 pages, 14 figures. Comments are welcom
Tunable quantum spin Hall effect in double quantum wells
The field of topological insulators (TIs) is rapidly growing. Concerning
possible applications, the search for materials with an easily controllable TI
phase is a key issue. The quantum spin Hall effect, characterized by a single
pair of helical edge modes protected by time-reversal symmetry, has been
demonstrated in HgTe-based quantum wells (QWs) with an inverted bandgap. We
analyze the topological properties of a generically coupled HgTe-based double
QW (DQW) and show how in such a system a TI phase can be driven by an
inter-layer bias voltage, even when the individual layers are non-inverted. We
argue, that this system allows for similar (layer-)pseudospin based physics as
in bilayer graphene but with the crucial absence of a valley degeneracy.Comment: 9 pages, 8 figures, extended version (accepted Phys. Rev. B
Fine structure of "zero-mode" Landau levels in HgTe/HgCdTe quantum wells
HgTe/HgCdTe quantum wells with the inverted band structure have been probed
using far infrared magneto-spectroscopy. Realistic calculations of Landau level
diagrams have been performed to identify the observed transitions.
Investigations have been greatly focused on the magnetic field dependence of
the peculiar pair of "zero-mode" Landau levels which characteristically split
from the upper conduction and bottom valence bands, and merge under the applied
magnetic field. The observed avoided crossing of these levels is tentatively
attributed to the bulk inversion asymmetry of zinc blend compounds.Comment: 5 pages, 4 figure
Magnetotransport in Double Quantum Well with Inverted Energy Spectrum: HgTe/CdHgTe
We present the first experimental study of the double-quantum-well (DQW)
system made of 2D layers with inverted energy band spectrum: HgTe. The
magnetotransport reveals a considerably larger overlap of the conduction and
valence subbands than in known HgTe single quantum wells (QW), which may be
regulated by an applied gate voltage . This large overlap manifests itself
in a much higher critical field separating the range above it where the
quantum peculiarities shift linearly with and the range below with a
complicated behavior. In the latter case the -shaped and double--shaped
structures in the Hall magnetoresistance are observed with their
scale in field pronouncedly enlarged as compared to the pictures observed in an
analogous single QW. The coexisting electrons and holes were found in the whole
investigated range of positive and negative as revealed from fits to the
low-field -shaped and from the Fourier analysis of
oscillations in . A peculiar feature here is that the found
electron density remains almost constant in the whole range of investigated
while the hole density drops down from the value a factor of 6 larger
than at extreme negative to almost zero at extreme positive
passing through the charge neutrality point. We show that this difference
between and stems from an order of magnitude larger density of states
for holes in the lateral valence band maxima than for electrons in the
conduction band minimum. We interpret the observed reentrant sign-alternating
between electronic and hole conductivities and its zero
resistivity state in the quantum Hall range of fields on the basis of a
calculated picture of magnetic levels in a DQW.Comment: 15 pages, 13 figure
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