6,579 research outputs found
Topological Superconductivity Intertwined with Broken Symmetries
Recently the superconductor and topological semimetal PbTaSe was
experimentally found to exhibit surface-only lattice rotational symmetry
breaking below . We exploit the Ginzburg-Landau free energy and propose a
microscopic two-channel model to study possible superconducting states on the
surface of PbTaSe. We identify two types of topological superconducting
states. One is time-reversal invariant and preserves the lattice hexagonal
symmetry while the other breaks both symmetries. We find that such
time-reversal symmetry breaking is unavoidable for a superconducting state in a
two dimensional irreducible representation of crystal point group in a system
where the spatial inversion symmetry is broken and the strong spin-orbit
coupling is present. Our findings will guide the search for topological chiral
superconductors.Comment: 4+5 pages, 5 figure
Floquet topological insulator phase in a Weyl semimetal thin film with disorder
We investigate the effects of periodic fields and disorder on topological
properties of a Weyl-semimetal thin film. The two periodic fields, i.e., a
periodic magnetic field and elliptically polarized light, are discussed
respectively. By use of the Floquet theory, we find that both the two periodic
drives can resonantly induce the topological transitions from normal insulator
(NI) phases to Floquet topological insulator (FTI) phases. The Floquet
topological transitions are characterized by variation of Chern number.
Moreover, we show that the Floquet topological transitions can be explained by
a combination of the quantum well approximation and the rotating wave
approximation. In the disordered Weyl-semimetal thin film model under periodic
fields, we calculate the Bott index to characterize topological phase. It is
found that the FTI phase is robust against weak disorder, and collapses for
strong disorder strength. Interestingly, we find that disorder can also induce
a topological transition from a topological trivial phase to an FTI phase,
establishing the Floquet topological Anderson insulator (FTAI) phase. Finally,
an effective-medium theory based on the Born approximation further confirms the
numerical conclusions
Topological Anderson insulator phase in a Dirac-semimetal thin film
The recently discovered topological Dirac semimetal represents a new exotic
quantum state of matter. Topological Dirac semimetals can be viewed as three
dimensional analogues of graphene, in which the Dirac nodes are protected by
crystalline symmetry. It has been found that quantum confinement effect can gap
out Dirac nodes and convert Dirac semimetal to a band insulator. The band
insulator is either normal insulator or quantum spin Hall insulator depending
on the thin film thickness. We present the study of disorder effects in thin
film of Dirac semimetals. It is found that moderate Anderson disorder strength
can drive a topological phase transition from normal band insulator to
topological Anderson insulator in Dirac semimetal thin film. The numerical
calculation based on the model parameters of Dirac semimetal NaBi shows
that in the topological Anderson insulator phase a quantized conductance
plateau occurs in the bulk gap of band insulator, and the distributions of
local currents further confirm that the quantized conductance plateau arises
from the helical edge states induced by disorder. Finally, an effective medium
theory based on Born approximation fits the numerical data
Disorder-induced topological phase transitions on Lieb lattices
Motivated by the very recent experimental realization of electronic Lieb
lattices and research interest on topological states of matter, we study the
topological phase transitions driven by Anderson disorder on spin-orbit coupled
Lieb lattices in the presence of spin-independent and dependent potentials. By
combining the numerical transport and self-consistent Born approximation
methods, we found that both time-reversal invariant and broken Lieb lattices
can host disorder-induced gapful topological phases, including the quantum spin
Hall insulator (QSHI) and quantum anomalous Hall insulator (QAHI) phases. For
the time-reversal invariant case, this disorder can induce a topological phase
transition directly from normal insulator (NI) to the QSHI. While for the
time-reversal broken case, the disorder can induce either a QAHI-QSHI phase
transition or a NI-QAHI-QSHI phase transition. Remarkably, the time-reversal
broken QSHI phase can be induced by Anderson disorder on the spin-orbit coupled
Lieb lattices without time-reversal symmetry.Comment: accepted for publication in Phys. Rev.
N-Version Obfuscation: Impeding Software Tampering Replication with Program Diversity
Tamper-resistance is a fundamental software security research area. Many
approaches have been proposed to thwart specific procedures of tampering, e.g.,
obfuscation and self-checksumming. However, to our best knowledge, none of them
can achieve theoretically tamper-resistance. Our idea is to impede the
replication of tampering via program diversification, and thus increasing the
complexity to break the whole software system. To this end, we propose to
deliver same featured, but functionally nonequivalent software copies to
different machines. We formally define the problem as N-version obfuscation,
and provide a viable means to solve the problem. Our evaluation result shows
that the time required for breaking a software system is linearly increased
with the number of software versions, which is O(n) complexity
The effect of in-plane magnetic field and applied strain in quantum spin Hall systems: application to InAs/GaSb quantum wells
Motivated by the recent discovery of quantized spin Hall effect in InAs/GaSb
quantum wells\cite{du2013}\cite{xu2014}, we theoretically study the effects
of in-plane magnetic field and strain effect to the quantization of charge
conductance by using Landauer-Butikker formalism. Our theory predicts a
robustness of the conductance quantization against the magnetic field up to a
very high field of 20 tesla. We use a disordered hopping term to model the
strain and show that the strain may help the quantization of the conductance.
Relevance to the experiments will be discussed.Comment: 8 pages, 10 figures. Comments are welcome
PersisDroid: Android Performance Diagnosis via Anatomizing Asynchronous Executions
Android applications (apps) grow dramatically in recent years. Apps are user
interface (UI) centric typically. Rapid UI responsiveness is key consideration
to app developers. However, we still lack a handy tool for profiling app
performance so as to diagnose performance problems. This paper presents
PersisDroid, a tool specifically designed for this task. The key notion of
PersisDroid is that the UI-triggered asynchronous executions also contribute to
the UI performance, and hence its performance should be properly captured to
facilitate performance diagnosis. However, Android allows tremendous ways to
start the asynchronous executions, posing a great challenge to profiling such
execution. This paper finds that they can be grouped into six categories. As a
result, they can be tracked and profiled according to the specifics of each
category with a dynamic instrumentation approach carefully tailored for
Android. PersisDroid can then properly profile the asynchronous executions in
task granularity, which equips it with low-overhead and high compatibility
merits. Most importantly, the profiling data can greatly help the developers in
detecting and locating performance anomalies. We code and open-source release
PersisDroid. The tool is applied in diagnosing 20 open-source apps, and we find
11 of them contain potential performance problems, which shows its
effectiveness in performance diagnosis for Android apps
On Secure and Usable Program Obfuscation: A Survey
Program obfuscation is a widely employed approach for software intellectual
property protection. However, general obfuscation methods (e.g., lexical
obfuscation, control obfuscation) implemented in mainstream obfuscation tools
are heuristic and have little security guarantee. Recently in 2013, Garg et al.
have achieved a breakthrough in secure program obfuscation with a graded
encoding mechanism and they have shown that it can fulfill a compelling
security property, i.e., indistinguishability. Nevertheless, the mechanism
incurs too much overhead for practical usage. Besides, it focuses on
obfuscating computation models (e.g., circuits) rather than real codes. In this
paper, we aim to explore secure and usable obfuscation approaches from the
literature. Our main finding is that currently we still have no such approaches
made secure and usable. The main reason is we do not have adequate evaluation
metrics concerning both security and performance. On one hand, existing
code-oriented obfuscation approaches generally evaluate the increased obscurity
rather than security guarantee. On the other hand, the performance requirement
for model-oriented obfuscation approaches is too weak to develop practical
program obfuscation solutions
Theory for Spin Selective Andreev Reflection in Vortex Core of Topological Superconductor: Majorana Zero Modes on Spherical Surface and Application to Spin Polarized Scanning Tunneling Microscope Probe
Majorana zero modes (MZMs) have been predicted to exist in the topological
insulator (TI)/superconductor (SC) heterostructure. Recent spin polarized
scanning tunneling microscope (STM) experiment has observed
spin-polarization dependence of the zero bias differential tunneling
conductance at the center of vortex core, which may be attributed to the spin
selective Andreev reflection, a novel property of the MZMs theoretically
predicted in 1-dimensional nanowire. Here we consider a helical electron
system described by a Rashba spin orbit coupling Hamiltonian on a spherical
surface with a s-wave superconducting pairing due to proximity effect. We
examine in-gap excitations of a pair of vortices with one at the north pole and
the other at the south pole. While the MZM is not a spin eigenstate, the spin
wavefunction of the MZM at the center of the vortex core, r = 0, is parallel to
the magnetic field, and the local Andreev reflection of the MZM is spin
selective, namely occurs only when the STM tip has the spin polarization
parallel to the magnetic field, similar to the case in 1-dimensional nanowire2.
The total local differential tunneling conductance consists of the normal term
proportional to the local density of states and an additional term arising from
the Andreev reflection. We also discuss the finite size effect, for which the
MZM at the north pole is hybridized with the MZM at the south pole. We apply
our theory to examine the recently reported spin-polarized STM experiments and
show good agreement with the experiments.Comment: 14 pages, 14 figures, 1 table. Comments are welcome
Quantum coherence of double-well BEC: a SU(2)-coherent-state path-integral approach
Macroscopic quantum coherence of Bose gas in a double-well potential is
studied based on SU(2)-coherent-state path-integral. The ground state and
fluctuations around it can be obtained by this method. In this picture, one can
obtain macroscopic quantum superposition states for attractive Bose gas. The
coherent gap of degenerate ground states is obtained with the instanton
technique. The phenomenon of macroscopic quantum self-trapping is also
discussed.Comment: 6 pages, 2 figures, final version to appear in Physcial Review
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