143 research outputs found
Topological Anderson Insulator in Three Dimensions
Disorder, ubiquitously present in solids, is normally detrimental to the
stability of ordered states of matter. In this letter we demonstrate that not
only is the physics of a strong topological insulator robust to disorder but,
remarkably, under certain conditions disorder can become fundamentally
responsible for its existence. We show that disorder, when sufficiently strong,
can transform an ordinary metal with strong spin-orbit coupling into a strong
topological `Anderson' insulator, a new topological phase of quantum matter in
three dimensions.Comment: 5 pages, 2 figures. For related work and info visit
http://www.physics.ubc.ca/~franz
Dissipation-driven quantum phase transition in superconductor-graphene systems
We show that a system of Josephson junctions coupled via low-resistance
tunneling contacts to graphene substrate(s) may effectively operate as a
current switching device. The effect is based on the dissipation-driven
superconductor-to-insulator quantum phase transition, which happens due to the
interplay of the Josephson effect and Coulomb blockade. Coupling to a graphene
substrate with gapless excitations further enhances charge fluctuations
favoring superconductivity. The effect is shown to scale exponentially with the
Fermi energy in graphene, which can be controlled by the gate voltage. We
develop a theory, which quantitatively describes the quantum phase transition
in a two-dimensional Josephson junction array, but it is expected to provide a
reliable qualitative description for one-dimensional systems as well. We argue
that the local effect of dissipation-induced enhancement of superconductivity
is very robust and a similar sharp crossover should be present in finite
Josephson junction chains.Comment: 4 pages, 3 figure
Simulation results for an interacting pair of resistively shunted Josephson junctions
Using a new cluster Monte Carlo algorithm, we study the phase diagram and
critical properties of an interacting pair of resistively shunted Josephson
junctions. This system models tunneling between two electrodes through a small
superconducting grain, and is described by a double sine-Gordon model. In
accordance with theoretical predictions, we observe three different phases and
crossover effects arising from an intermediate coupling fixed point. On the
superconductor-to-metal phase boundary, the observed critical behavior is
within error-bars the same as in a single junction, with identical values of
the critical resistance and a correlation function exponent which depends only
on the strength of the Josephson coupling. We explain these critical properties
on the basis of a renormalization group (RG) calculation. In addition, we
propose an alternative new mean-field theory for this transition, which
correctly predicts the location of the phase boundary at intermediate Josephson
coupling strength.Comment: 21 pages, some figures best viewed in colo
Ettingshausen effect due to Majorana modes
The presence of Majorana zero-energy modes at vortex cores in a topological
superconductor implies that each vortex carries an extra entropy , given
by , that is independent of temperature. By utilizing this
special property of Majorana modes, the edges of a topological superconductor
can be cooled (or heated) by the motion of the vortices across the edges. As
vortices flow in the transverse direction with respect to an external imposed
supercurrent, due to the Lorentz force, a thermoelectric effect analogous to
the Ettingshausen effect is expected to occur between opposing edges. We
propose an experiment to observe this thermoelectric effect, which could
directly probe the intrinsic entropy of Majorana zero-energy modes.Comment: 16 pages, 3 figure
Vortices and the entrainment transition in the 2D Kuramoto model
We study synchronization in the two-dimensional lattice of coupled phase
oscillators with random intrinsic frequencies. When the coupling is larger
than a threshold , there is a macroscopic cluster of
frequency-synchronized oscillators. We explain why the macroscopic cluster
disappears at . We view the system in terms of vortices, since cluster
boundaries are delineated by the motion of these topological defects. In the
entrained phase (), vortices move in fixed paths around clusters, while
in the unentrained phase (), vortices sometimes wander off. These
deviant vortices are responsible for the disappearance of the macroscopic
cluster. The regularity of vortex motion is determined by whether clusters
behave as single effective oscillators. The unentrained phase is also
characterized by time-dependent cluster structure and the presence of chaos.
Thus, the entrainment transition is actually an order-chaos transition. We
present an analytical argument for the scaling for small
lattices, where is the threshold for phase-locking. By also deriving the
scaling , we thus show that for small , in
agreement with numerics. In addition, we show how to use the linearized model
to predict where vortices are generated.Comment: 11 pages, 8 figure
Strong-disorder renormalization for interacting non-Abelian anyon systems in two dimensions
We consider the effect of quenched spatial disorder on systems of
interacting, pinned non-Abelian anyons as might arise in disordered Hall
samples at filling fractions \nu=5/2 or \nu=12/5. In one spatial dimension,
such disordered anyon models have previously been shown to exhibit a hierarchy
of infinite randomness phases. Here, we address systems in two spatial
dimensions and report on the behavior of Ising and Fibonacci anyons under the
numerical strong-disorder renormalization group (SDRG). In order to manage the
topology-dependent interactions generated during the flow, we introduce a
planar approximation to the SDRG treatment. We characterize this planar
approximation by studying the flow of disordered hard-core bosons and the
transverse field Ising model, where it successfully reproduces the known
infinite randomness critical point with exponent \psi ~ 0.43. Our main
conclusion for disordered anyon models in two spatial dimensions is that
systems of Ising anyons as well as systems of Fibonacci anyons do not realize
infinite randomness phases, but flow back to weaker disorder under the
numerical SDRG treatment.Comment: 12 pages, 12 figures, 1 tabl
Weber blockade theory of magnetoresistance oscillations in superconducting strips
Recent experiments on the conductance of thin, narrow superconducting strips
have found periodic fluctuations, as a function of the perpendicular magnetic
field, with a period corresponding to approximately two flux quanta per strip
area [A. Johansson et al., Phys. Rev. Lett. {\bf 95}, 116805 (2005)]. We argue
that the low-energy degrees of freedom responsible for dissipation correspond
to vortex motion. Using vortex/charge duality, we show that the superconducting
strip behaves as the dual of a quantum dot, with the vortices, magnetic field,
and bias current respectively playing the roles of the electrons, gate voltage
and source-drain voltage. In the bias-current vs. magnetic-field plane, the
strip conductance displays what we term `Weber blockade' diamonds, with vortex
conductance maxima (i.e., electrical resistance maxima) that, at small
bias-currents, correspond to the fields at which strip states of and
vortices have equal energy.Comment: 4+a bit pages, 3 figures, 1 tabl
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