73,941 research outputs found

### Superconducting proximity effect and Majorana fermions at the surface of a topological insulator

We study the proximity effect between an s-wave superconductor and the
surface states of a strong topological insulator. The resulting two dimensional
state resembles a spinless p_x+ip_y superconductor, but does not break time
reversal symmetry. This state supports Majorana bound states at vortices. We
show that linear junctions between superconductors mediated by the topological
insulator form a non chiral 1 dimensional wire for Majorana fermions, and that
circuits formed from these junctions provide a method for creating,
manipulating and fusing Majorana bound states.Comment: 4 pages, 3 figures, published versio

### Spin content of Lambda and its longitudinal polarization in $e^+e^-$ annihilation at high energies

Longitudinal polarization of Lambda produced in $e^+e^-$ annihilation at LEP
energies is calculated in a picture for the spin content of Lambda which is
consistent with the polarized deep inelastic lepton-nucleon scattering data and
SU(3) flavor symmetry for hyperon decay so that the spin of Lambda is not
completely carried by its $s$-valence quark. A comparison with the recent ALEPH
data and the results of earlier calculations based on the static quark model in
which the spin of Lambda is completely determined by the $s$-quark is given.
The result shows that further measurements of such polarization should provide
useful information to the question of which picture is more suitable in
describing the spin effects in the fragmentation processes.Comment: 12 pages, 2 figure

### Single spin asymmetries in inclusive high energy hadron-hadron collision processes

It has been realized for quite a long time that single-spin experiments, in
which one of the colliding objects is transversely polarized, can be helpful in
studying the properties of strong interaction in general and in testing Quantum
Chromodynamics (QCD) in particular.
Striking effects have been observed in the past few years which deviate
drastically from the expectation of the perturbative QCD parton model.
These effects have received much attention.
New experiments of the similar type are underway and/or planned.
Different theoretical attempts have been made to understand these effects.
In this review, the special role played by singly polarized high-energy
hadron-hadron collisions in High Energy Spin Physics is emphasized.
Characteristics of the available data for inclusive hadron productions are
briefly summarized.
Different theoretical approaches for such processes are reviewed with special
attention to a non-perturbative model which explicitly takes the orbital motion
of the valence quarks and hadronic surface effects into account.
The connection between such asymmetries and hyperon polarization in
unpolarized reactions is discussed.
An example of the possible application of such experimental results in other
processes is given.Comment: 62 pages, 17 ps-figures (Review article to appear in Inter. J. Mod.
Phys. A

### Time-dependent quantum transport and power-law decay of the transient current in a nano-relay and nano-oscillator

Time-dependent nonequilibrium Green's functions are used to study electron
transport properties in a device consisting of two linear chain leads and a
time-dependent interleads coupling that is switched on non-adiabatically. We
derive a numerically exact expression for the particle current and examine its
characteristics as it evolves in time from the transient regime to the
long-time steady-state regime. We find that just after switch-on the current
initially overshoots the expected long-time steady-state value, oscillates and
decays as a power law, and eventually settles to a steady-state value
consistent with the value calculated using the Landauer formula. The power-law
parameters depend on the values of the applied bias voltage, the strength of
the couplings, and the speed of the switch-on. In particular, the oscillating
transient current decays away longer for lower bias voltages. Furthermore, the
power-law decay nature of the current suggests an equivalent series
resistor-inductor-capacitor circuit wherein all of the components have
time-dependent properties. Such dynamical resistive, inductive, and capacitive
influences are generic in nano-circuites where dynamical switches are
incorporated. We also examine the characteristics of the dynamical current in a
nano-oscillator modeled by introducing a sinusoidally modulated interleads
coupling between the two leads. We find that the current does not strictly
follow the sinusoidal form of the coupling. In particular, the maximum current
does not occur during times when the leads are exactly aligned. Instead, the
times when the maximum current occurs depend on the values of the bias
potential, nearest-neighbor coupling, and the interleads coupling.Comment: version accepted for publication in JA

### Anomalous Supercurrent from Majorana States in Topological Insulator Josephson Junctions

We propose a Josephson junction setup based on a topological insulator (TI)
thin film to detect Majorana states, which exploits the unique helical and
extended nature of the TI surface state. When the magnetic flux through the
junction is close to an integer number of flux quanta, Majorana states, present
on both surfaces of the film, give rise to a narrow peak-dip structure in the
current- phase relation by hybridizing at the edge of the junction. Moreover,
the maximal Majorana-state contribution to Josephson current takes a (nearly)
universal value, approximately equal to the supercurrent capacity of a single
quantum-channel. These features provide a characteristic signature of Majorana
states based entirely on supercurrent.Comment: 6 pages, 5 figure

### TOFEC: Achieving Optimal Throughput-Delay Trade-off of Cloud Storage Using Erasure Codes

Our paper presents solutions using erasure coding, parallel connections to
storage cloud and limited chunking (i.e., dividing the object into a few
smaller segments) together to significantly improve the delay performance of
uploading and downloading data in and out of cloud storage.
TOFEC is a strategy that helps front-end proxy adapt to level of workload by
treating scalable cloud storage (e.g. Amazon S3) as a shared resource requiring
admission control. Under light workloads, TOFEC creates more smaller chunks and
uses more parallel connections per file, minimizing service delay. Under heavy
workloads, TOFEC automatically reduces the level of chunking (fewer chunks with
increased size) and uses fewer parallel connections to reduce overhead,
resulting in higher throughput and preventing queueing delay. Our trace-driven
simulation results show that TOFEC's adaptation mechanism converges to an
appropriate code that provides the optimal delay-throughput trade-off without
reducing system capacity. Compared to a non-adaptive strategy optimized for
throughput, TOFEC delivers 2.5x lower latency under light workloads; compared
to a non-adaptive strategy optimized for latency, TOFEC can scale to support
over 3x as many requests

### Quantum Tunneling of Spin Particles in Periodic Potentials with Asymmetric Twin Barriers

The tunneling effect of a periodic potential with an asymmetric twin barrier
per period is calculated using the instanton method. The model is derived from
the Hamiltonian of a small ferromagnetic particle in an external magnetic field
using the spin-coherent-state path integral. The instantons in two neighbouring
barriers differ and lead to different level shifts $\triangle\epsilon_1,
\triangle\epsilon_2$. We derive with Bloch theory the energy spectrum which has
formally the structure of an energy band. The spectrum depends on both level
shifts. The removal of Kramer's degeneracy by an external magnetic field is
discussed. In addition we find a new kind of quenching of macroscopic quantum
coherence which is irrelevant to Kramer's degeneracy.Comment: 18 pages, LaTex, one figur

- …