85 research outputs found
Interplay of exciton condensation and quantum spin Hall effect in InAs/GaSb bilayers
We study the phase diagram of the inverted InAs/GaSb bilayer quantum wells.
For small tunneling amplitude between the layers, we find that the system is
prone to formation of an s-wave exciton condensate phase, where the
spin-structure of the order parameter is uniquely determined by the small
spin-orbit coupling arising from the bulk inversion asymmetry. The phase is
topologically trivial and does not support edge transport. On the contrary, for
large tunneling amplitude, we obtain a topologically non-trivial quantum spin
Hall insulator phase with a p-wave exciton order parameter, which enhances the
hybridization gap. These topologically distinct insulators are separated by an
insulating phase with spontaneously broken time-reversal symmetry. Close to the
phase transition between the quantum spin Hall and time-reversal broken phases,
the edge transport shows quantized conductance in small samples, whereas in
long samples the mean free path associated with the backscattering at the edge
is temperature independent, in agreement with recent experiments.Comment: v. 2, 9 pages, 5 figure
Extended topological group structure due to average reflection symmetry
We extend the single-particle topological classification of insulators and
superconductors to include systems in which disorder preserves average
reflection symmetry. We show that the topological group structure of bulk
Hamiltonians and topological defects is exponentially extended when this
additional condition is met, and examine some of its physical consequences.
Those include localization-delocalization transitions between topological
phases with the same boundary conductance, as well as gapless topological
defects stabilized by average reflection symmetry.Comment: 8 pages, 5 figures, 1 table; improved section 4 "Extended topological
classification" incl. example of stacked QSH layer
X-shaped and Y-shaped Andreev resonance profiles in a superconducting quantum dot
The quasi-bound states of a superconducting quantum dot that is weakly
coupled to a normal metal appear as resonances in the Andreev reflection
probability, measured via the differential conductance. We study the evolution
of these Andreev resonances when an external parameter (such as magnetic field
or gate voltage) is varied, using a random-matrix model for the
scattering matrix. We contrast the two ensembles with broken time-reversal
symmetry, in the presence or absence of spin-rotation symmetry (class C or D).
The poles of the scattering matrix in the complex plane, encoding the center
and width of the resonance, are repelled from the imaginary axis in class C. In
class D, in contrast, a number of the poles has zero real
part. The corresponding Andreev resonances are pinned to the middle of the gap
and produce a zero-bias conductance peak that does not split over a range of
parameter values (Y-shaped profile), unlike the usual conductance peaks that
merge and then immediately split (X-shaped profile).Comment: Contribution for the JETP special issue in honor of A.F. Andreev's
75th birthday. 9 pages, 8 figure
Bimodal conductance distribution of Kitaev edge modes in topological superconductors
A two-dimensional superconductor with spin-triplet p-wave pairing supports
chiral or helical Majorana edge modes with a quantized (length -independent)
thermal conductance. Sufficiently strong anisotropy removes both chirality and
helicity, doubling the conductance in the clean system and imposing a
super-Ohmic decay in the presence of disorder. We explain the
absence of localization in the framework of the Kitaev Hamiltonian, contrasting
the edge modes of the two-dimensional system with the one-dimensional Kitaev
chain. While the disordered Kitaev chain has a log-normal conductance
distribution peaked at an exponentially small value, the Kitaev edge has a
bimodal distribution with a second peak near the conductance quantum. Shot
noise provides an alternative, purely electrical method of detection of these
charge-neutral edge modes.Comment: 11 pages, 13 figure
Phase-locked magnetoconductance oscillations as a probe of Majorana edge states
We calculate the Andreev conductance of a superconducting ring interrupted by
a flux-biased Josephson junction, searching for electrical signatures of
circulating edge states. Two-dimensional pair potentials of spin-singlet d-wave
and spin-triplet p-wave symmetry support, respectively, (chiral) Dirac modes
and (chiral or helical) Majorana modes. These produce h/e-periodic
magnetoconductance oscillations of amplitude \simeq (e^{2}/h)N^{-1/2}, measured
via an N-mode point contact at the inner or outer perimeter of the grounded
ring. For Dirac modes the oscillations in the two contacts are independent,
while for an unpaired Majorana mode they are phase locked by a topological
phase transition at the Josephson junction.Comment: 10 pages, 6 figures. New appendix on the gauge invariant
discretization of the Bogoliubov-De Gennes equation. Accepted for publication
in PR
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