2,314 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
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
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
Photoresist patterned thick-film piezoelectric elements on silicon
A fundamental limitation of screen printing is the achievable alignment accuracy and resolution. This paper presents details of a thick-resist process that improves both of these factors. The technique involves exposing/developing a thick resist to form the desired pattern and then filling the features with thick film material using a doctor blading process. Registration accuracy comparable with standard photolithographic processes has been achieved resulting in minimum feature sizes of <50 ?m and a film thickness of 100 ?m. Piezoelectric elements have been successfully poled on a platinised silicon wafer with a measured d 33 value of 60 pCN?1
An almost Poisson structure for the generalized rigid body equations
In this paper we introduce almost Poisson structures on Lie groups which
generalize Poisson structures based on the use of the classical Yang-Baxter identity.
Almost Poisson structures fail to be Poisson structures in the sense that they do
not satisfy the Jacobi identity.In the case of cross products of Lie groups, we show
that an almost Poisson structure can be used to derive a system which is intimately
related to a fundamental Hamiltonian integrable system — the generalized rigid body
equations
Microelectromechanical systems vibration powered electromagnetic generator for wireless sensor applications
This paper presents a silicon microgenerator, fabricated using standard silicon micromachining techniques, which converts external ambient vibrations into electrical energy. Power is generated by an electromagnetic transduction mechanism with static magnets positioned on either side of a moving coil, which is located on a silicon structure designed to resonate laterally in the plane of the chip. The volume of this device is approximately 100 mm3. ANSYS finite element analysis (FEA) has been used to determine the optimum geometry for the microgenerator. Electromagnetic FEA simulations using Ansoft’s Maxwell 3D software have been performed to determine the voltage generated from a single beam generator design. The predicted voltage levels of 0.7–4.15 V can be generated for a two-pole arrangement by tuning the damping factor to achieve maximum displacement for a given input excitation. Experimental results from the microgenerator demonstrate a maximum power output of 104 nW for 0.4g (g=9.81 m s1) input acceleration at 1.615 kHz. Other frequencies can be achieved by employing different geometries or material
Discrete rigid body dynamics and optimal control
We analyze an alternative formulation of the rigid body equations, their relationship with the discrete rigid body equations of Moser-Veselov (1991) and their formulation as an optimal control problem. In addition we discuss a general class of discrete optimal control problems
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
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