2,158 research outputs found
Effects of electron scattering on the topological properties of nanowires: Majorana fermions from disorder and superlattices
We focus on inducing topological state from regular, or irregular scattering
in (i) p-wave superconducting wires and (ii) Rashba wires proximity coupled to
an s-wave superconductor. We find that contrary to common expectations the
topological properties of both systems are fundamentally different: In p-wave
wires, disorder generally has a detrimental effect on the topological order and
the topological state is destroyed beyond a critical disorder strength. In
contrast, in Rashba wires, which are relevant for recent experiments, disorder
can {\it induce} topological order, reducing the need for quasiballistic
samples to obtain Majorana fermions. Moreover, we find that the total phase
space area of the topological state is conserved for long disordered Rashba
wires, and can even be increased in an appropriately engineered superlattice
potential.Comment: 5 pages, 3 figs, RevTe
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
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
Interfaces Within Graphene Nanoribbons
We study the conductance through two types of graphene nanostructures:
nanoribbon junctions in which the width changes from wide to narrow, and curved
nanoribbons. In the wide-narrow structures, substantial reflection occurs from
the wide-narrow interface, in contrast to the behavior of the much studied
electron gas waveguides. In the curved nanoribbons, the conductance is very
sensitive to details such as whether regions of a semiconducting armchair
nanoribbon are included in the curved structure -- such regions strongly
suppress the conductance. Surprisingly, this suppression is not due to the band
gap of the semiconducting nanoribbon, but is linked to the valley degree of
freedom. Though we study these effects in the simplest contexts, they can be
expected to occur for more complicated structures, and we show results for
rings as well. We conclude that experience from electron gas waveguides does
not carry over to graphene nanostructures. The interior interfaces causing
extra scattering result from the extra effective degrees of freedom of the
graphene structure, namely the valley and sublattice pseudospins.Comment: 19 pages, published version, several references added, small changes
to conclusion
Strokes2Surface: Recovering Curve Networks From 4D Architectural Design Sketches
We present Strokes2Surface, an offline geometry reconstruction pipeline that
recovers well-connected curve networks from imprecise 4D sketches to bridge
concept design and digital modeling stages in architectural design. The input
to our pipeline consists of 3D strokes' polyline vertices and their timestamps
as the 4th dimension, along with additional metadata recorded throughout
sketching. Inspired by architectural sketching practices, our pipeline combines
a classifier and two clustering models to achieve its goal. First, with a set
of extracted hand-engineered features from the sketch, the classifier
recognizes the type of individual strokes between those depicting boundaries
(Shape strokes) and those depicting enclosed areas (Scribble strokes). Next,
the two clustering models parse strokes of each type into distinct groups, each
representing an individual edge or face of the intended architectural object.
Curve networks are then formed through topology recovery of consolidated Shape
clusters and surfaced using Scribble clusters guiding the cycle discovery. Our
evaluation is threefold: We confirm the usability of the Strokes2Surface
pipeline in architectural design use cases via a user study, we validate our
choice of features via statistical analysis and ablation studies on our
collected dataset, and we compare our outputs against a range of
reconstructions computed using alternative methods.Comment: 15 pages, 14 figure
One-loop surface tensions of (supersymmetric) kink domain walls from dimensional regularization
We consider domain walls obtained by embedding the 1+1-dimensional
-kink in higher dimensions. We show that a suitably adapted dimensional
regularization method avoids the intricacies found in other regularization
schemes in both supersymmetric and non-supersymmetric theories. This method
allows us to calculate the one-loop quantum mass of kinks and surface tensions
of kink domain walls in a very simple manner, yielding a compact d-dimensional
formula which reproduces many of the previous results in the literature. Among
the new results is the nontrivial one-loop correction to the surface tension of
a 2+1 dimensional N=1 supersymmetric kink domain wall with chiral domain-wall
fermions.Comment: 23 pages, LATeX; v2: 25 pages, 2 references added, extended
discussion of renormalization schemes which dispels apparent contradiction
with previous result
Wigner-Poisson statistics of topological transitions in a Josephson junction
The phase-dependent bound states (Andreev levels) of a Josephson junction can
cross at the Fermi level, if the superconducting ground state switches between
even and odd fermion parity. The level crossing is topologically protected, in
the absence of time-reversal and spin-rotation symmetry, irrespective of
whether the superconductor itself is topologically trivial or not. We develop a
statistical theory of these topological transitions in an N-mode quantum-dot
Josephson junction, by associating the Andreev level crossings with the real
eigenvalues of a random non-Hermitian matrix. The number of topological
transitions in a 2pi phase interval scales as sqrt(N) and their spacing
distribution is a hybrid of the Wigner and Poisson distributions of
random-matrix theory.Comment: 12 pages, 15 figures; v2 to appear in PRL, with appendix in the
supplementary materia
Biexciton recombination rates in self-assembled quantum dots
The radiative recombination rates of interacting electron-hole pairs in a
quantum dot are strongly affected by quantum correlations among electrons and
holes in the dot. Recent measurements of the biexciton recombination rate in
single self-assembled quantum dots have found values spanning from two times
the single exciton recombination rate to values well below the exciton decay
rate. In this paper, a Feynman path-integral formulation is developed to
calculate recombination rates including thermal and many-body effects. Using
real-space Monte Carlo integration, the path-integral expressions for realistic
three-dimensional models of InGaAs/GaAs, CdSe/ZnSe, and InP/InGaP dots are
evaluated, including anisotropic effective masses. Depending on size, radiative
rates of typical dots lie in the regime between strong and intermediate
confinement. The results compare favorably to recent experiments and
calculations on related dot systems. Configuration interaction calculations
using uncorrelated basis sets are found to be severely limited in calculating
decay rates.Comment: 11 pages, 4 figure
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