3,459 research outputs found
Search for Majorana fermions in multiband semiconducting nanowires
We study multiband semiconducting nanowires proximity-coupled with an s-wave
superconductor. We show that when odd number of subbands are occupied the
system realizes non-trivial topological state supporting Majorana modes
localized at the ends. We study the topological quantum phase transition in
this system and analytically calculate the phase diagram as a function of the
chemical potential and magnetic field. Our key finding is that multiband
occupancy not only lifts the stringent constraint of one-dimensionality but
also allows to have higher carrier density in the nanowire and as such
multisubband nanowires are better-suited for observing the Majorana particle.
We study the robustness of the topological phase by including the effects of
the short- and long-range disorder. We show that in the limit of strong
interband mixing there is an optimal regime in the phase diagram ("sweet spot")
where the topological state is to a large extent insensitive to the presence of
disorder.Comment: 4 pages, 3 figures, expanded version includes new results; accepted
for publication in PR
Dimensional crossover in spin-orbit-coupled semiconductor nanowires with induced superconducting pairing
We show that the topological Majorana modes in nanowires much longer than the
superconducting coherence length are adiabatically connected with discrete
zero-energy states generically occurring in short nanowires. We demonstrate
that these zero-energy crossings can be tuned by an external magnetic field and
are protected by the particle-hole symmetry. We study the evolution of the
low-energy spectrum and the splitting oscillations as a function of magnetic
field, wire length, and chemical potential, manifestly establishing that the
low-energy physics of short wires is related to that occurring in long wires.
This physics, which represents a hallmark of spinless p-wave superconductivity,
can be observed in tunneling conductance measurements.Comment: published version, 7 pages, 7 color figure
Soft superconducting gap in semiconductor-based Majorana nanowires
We develop a theory for the proximity effect in
superconductor-semiconductor-normal metal tunneling structures, which have
recently been extensively studied experimentally, leading to the observation of
transport signatures consistent with the predicted zero-energy Majorana bound
states. We show that our model for the semiconductor nanowire having multiple
occupied subbands with different transmission probabilities through the barrier
reproduces the observed "soft-gap" behavior associated with substantial subgap
tunneling conductance. We study the manifestations of the soft gap phenomenon
both in the tunneling conductance and in local density of states measurements
and discuss the correlations between these two quantities. We emphasize that
the proximity effect associated with the hybridization between low-lying states
in the multiband semiconductor and the normal metal states in the lead is an
intrinsic effect leading to the soft gap problem. In addition to the intrinsic
contribution, there may be extrinsic effects, such as, for example, interface
disorder, exacerbating the soft gap problem. Our work establishes the generic
possibility of an ubiquitous presence of an intrinsic soft gap in the
superconductor-semiconductor-normal metal tunneling transport conductance
induced by the inverse proximity effect of the normal metal.Comment: published version, 11+ pages, 8 figure
Strong Coupling Theory for Interacting Lattice Models
We develop a strong coupling approach for a general lattice problem. We argue
that this strong coupling perspective represents the natural framework for a
generalization of the dynamical mean field theory (DMFT). The main result of
this analysis is twofold: 1) It provides the tools for a unified treatment of
any non-local contribution to the Hamiltonian. Within our scheme, non-local
terms such as hopping terms, spin-spin interactions, or non-local Coulomb
interactions are treated on equal footing. 2) By performing a detailed
strong-coupling analysis of a generalized lattice problem, we establish the
basis for possible clean and systematic extensions beyond DMFT. To this end, we
study the problem using three different perspectives. First, we develop a
generalized expansion around the atomic limit in terms of the coupling
constants for the non-local contributions to the Hamiltonian. By analyzing the
diagrammatics associated with this expansion, we establish the equations for a
generalized dynamical mean-field theory (G-DMFT). Second, we formulate the
theory in terms of a generalized strong coupling version of the Baym-Kadanoff
functional. Third, following Pairault, Senechal, and Tremblay, we present our
scheme in the language of a perturbation theory for canonical fermionic and
bosonic fields and we establish the interpretation of various strong coupling
quantities within a standard perturbative picture.Comment: Revised Version, 17 pages, 5 figure
Proximity effect at the superconductor - topological insulator interface
We study the excitation spectrum of a topological insulator in contact with
an s-wave superconductor, starting from a microscopic model, and develop an
effective low-energy model for the proximity effect. In the vicinity of the
Dirac cone vertex, the effective model describing the states localized at the
interface is well approximated by a model of Dirac electrons experiencing
superconducting s-wave pairing. Away from the cone vertex, the induced pairing
potential develops a p-wave component with a magnitude sensitive to the
structure of the interface. Observing the induced s-wave superconductivity may
require tuning the chemical potential close to the Dirac point. Furthermore, we
find that the proximity of the superconductor leads to a significant
renormalization of the original parameters of the effective model describing
the surface states of a topological insulator.Comment: 4+ pages, 3 figures (published version
Non-equilibrium spin dynamics in a trapped Fermi gas with effective spin-orbit interaction
We consider a trapped atomic system in the presence of spatially varying
laser fields. The laser-atom interaction generates a pseudospin degree of
freedom (referred to simply as spin) and leads to an effective spin-orbit
coupling for the fermions in the trap. Reflections of the fermions from the
trap boundaries provide a physical mechanism for effective momentum relaxation
and non-trivial spin dynamics due to the emergent spin-orbit coupling. We
explicitly consider evolution of an initially spin-polarized Fermi gas in a
two-dimensional harmonic trap and derive non-equilibrium behavior of the spin
polarization. It shows periodic echoes with a frequency equal to the harmonic
trapping frequency. Perturbations, such as an asymmetry of the trap, lead to
the suppression of the spin echo amplitudes. We discuss a possible experimental
setup to observe spin dynamics and provide numerical estimates of relevant
parameters.Comment: 5 pages, 4 figures; published versio
Nodal/Antinodal Dichotomy and the Two Gaps of a Superconducting Doped Mott Insulator
We study the superconducting state of the hole-doped two-dimensional Hubbard
model using Cellular Dynamical Mean Field Theory, with the Lanczos method as
impurity solver. In the under-doped regime, we find a natural decomposition of
the one-particle (photoemission) energy-gap into two components. The gap in the
nodal regions, stemming from the anomalous self-energy, decreases with
decreasing doping. The antinodal gap has an additional contribution from the
normal component of the self-energy, inherited from the normal-state pseudogap,
and it increases as the Mott insulating phase is approached.Comment: Corrected typos, 4.5 pages, 4 figure
High-grade cervical dysplasia in pregnancy – psychological and medical challenges
Despite being rare, the incidence of pregnancy-related cancer is expected to rise as women continue to delay childbearing and give birth later in their reproductive years. In this broad category, tumors like breast cancer, dermatological neoplasia and cervical cancer are most common and tend to arise in women of childbearing age. All pregnant women with clinical and cytologic suspicion of cervical cancer, except for squamous atypia or low-grade squamous intraepithelial lesions, should undergo colposcopy, with or without biopsy, the latter being avoided if possible due to possible complications which, although rare, may involve preterm labor initiation.
Some studies have attempted to assimilate comparable results of USG with MRI during the gestational period by determining the sensitivity, specificity, and accuracy of trans-rectal ultrasound (TRUS) in comparison to magnetic resonance imaging (MRI). In order to identify the proper way to diagnose and treat the disease, because of the complexity due to pregnancy, a multidisciplinary team consisting of a gynecologist, medical and surgical oncologist, and radiologist should be assembled. Both maternal and fetal wellbeing should be taken into consideration when the medical team must choose among termination of pregnancy, delay of maternal treatment, and iatrogenic preterm delivery. Psychological counseling also plays an important role and due to the sensitivity of the issue, should continue through gestation and the postpartum.
In order to develop optimal guidelines for diagnosis, treatment, and outcome issues, large scale prospective studies are needed, but feasibility may be limited due to the scarcity of cervical cancer cases associated with pregnancy
Spin relaxation in a generic two-dimensional spin-orbit coupled system
We study the relaxation of a spin density injected into a two-dimensional
electron system with generic spin-orbit interactions. Our model includes the
Rashba as well as linear and cubic Dresselhaus terms. We explicitly derive a
general spin-charge coupled diffusion equation. Spin diffusion is characterized
by just two independent dimensionless parameters which control the interplay
between different spin-orbit couplings. The real-time representation of the
diffuson matrix (Green's function of the diffusion equation) is evaluated
analytically. The diffuson describes space-time dynamics of the injected spin
distribution. We explicitly study two regimes: The first regime corresponds to
negligible spin-charge coupling and is characterized by standard charge
diffusion decoupled from the spin dynamics. It is shown that there exist
several qualitatively different dynamic behaviors of the spin density, which
correspond to various domains in the spin-orbit coupling parameter space. We
discuss in detail a few interesting phenomena such as an enhancement of the
spin relaxation times, real space oscillatory dynamics, and anisotropic
transport. In the second regime, we include the effects of spin-charge
coupling. It is shown that the spin-charge coupling leads to an enhancement of
the effective charge diffusion coefficient. We also find that in the case of
strong spin-charge coupling, the relaxation rates formally become complex and
the spin/charge dynamics is characterized by real time oscillations. These
effects are qualitatively similar to those observed in spin-grating experiments
[Weber et al., Nature 437, 1330 (2005)].Comment: 18 pages, 7 figure
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