631 research outputs found
Disentanglement of two harmonic oscillators in relativistic motion
We study the dynamics of quantum entanglement between two Unruh-DeWitt
detectors, one stationary (Alice), and another uniformly accelerating (Rob),
with no direct interaction but coupled to a common quantum field in (3+1)D
Minkowski space. We find that for all cases studied the initial entanglement
between the detectors disappears in a finite time ("sudden death"). After the
moment of total disentanglement the correlations between the two detectors
remain nonzero until late times. The relation between the disentanglement time
and Rob's proper acceleration is observer dependent. The larger the
acceleration is, the longer the disentanglement time in Alice's coordinate, but
the shorter in Rob's coordinate.Comment: 16 pages, 8 figures; typos added, minor changes in Secs. I and
Quantum teleportation between moving detectors in a quantum field
We consider the quantum teleportation of continuous variables modeled by
Unruh-DeWitt detectors coupled to a common quantum field initially in the
Minkowski vacuum. An unknown coherent state of an Unruh-DeWitt detector is
teleported from one inertial agent (Alice) to an almost uniformly accelerated
agent (Rob, for relativistic motion), using a detector pair initially entangled
and shared by these two agents. The averaged physical fidelity of quantum
teleportation, which is independent of the observer's frame, always drops below
the best fidelity value from classical teleportation before the detector pair
becomes disentangled with the measure of entanglement evaluated around the
future lightcone of the joint measurement event by Alice. The distortion of the
quantum state of the entangled detector pair from the initial state can
suppress the fidelity significantly even when the detectors are still strongly
entangled around the lightcone. We point out that the dynamics of entanglement
of the detector pair observed in Minkowski frame or in quasi-Rindler frame are
not directly related to the physical fidelity of quantum teleportation in our
setup. These results are useful as a guide to making judicious choices of
states and parameter ranges and estimation of the efficiency of quantum
teleportation in relativistic quantum systems under environmental influences.Comment: 18 pages, 7 figure
Spanning trees on the Sierpinski gasket
We obtain the numbers of spanning trees on the Sierpinski gasket
with dimension equal to two, three and four. The general expression for the
number of spanning trees on with arbitrary is conjectured. The
numbers of spanning trees on the generalized Sierpinski gasket
with and are also obtained.Comment: 20 pages, 8 figures, 1 tabl
Graphene-based modulation-doped superlattice structures
The electronic transport properties of graphene-based superlattice structures
are investigated. A graphene-based modulation-doped superlattice structure
geometry is proposed and consist of periodically arranged alternate layers:
InAs/graphene/GaAs/graphene/GaSb. Undoped graphene/GaAs/graphene structure
displays relatively high conductance and enhanced mobilities at elevated
temperatures unlike modulation-doped superlattice structure more steady and
less sensitive to temperature and robust electrical tunable control on the
screening length scale. Thermionic current density exhibits enhanced behaviour
due to presence of metallic (graphene) mono-layers in superlattice structure.
The proposed superlattice structure might become of great use for new types of
wide-band energy gap quantum devices.Comment: 5 figure
Local structure study of In_xGa_(1-x)As semiconductor alloys using High Energy Synchrotron X-ray Diffraction
Nearest and higher neighbor distances as well as bond length distributions
(static and thermal) of the In_xGa_(1-x)As (0<x<1) semiconductor alloys have
been obtained from high real-space resolution atomic pair distribution
functions (PDFs). Using this structural information, we modeled the local
atomic displacements in In_xGa_(1-x)As alloys. From a supercell model based on
the Kirkwood potential, we obtained 3-D As and (In,Ga) ensemble averaged
probability distributions. This clearly shows that As atom displacements are
highly directional and can be represented as a combination of and
displacements. Examination of the Kirkwood model indicates that the standard
deviation (sigma) of the static disorder on the (In,Ga) sublattice is around
60% of the value on the As sublattice and the (In,Ga) atomic displacements are
much more isotropic than those on the As sublattice. The single crystal diffuse
scattering calculated from the Kirkwood model shows that atomic displacements
are most strongly correlated along directions.Comment: 10 pages, 12 figure
The Spectrum of Goldstini and Modulini
When supersymmetry is broken in multiple sectors via independent dynamics,
the theory furnishes a corresponding multiplicity of "goldstini" degrees of
freedom which may play a substantial role in collider phenomenology and
cosmology. In this paper, we explore the tree-level mass spectrum of goldstini
arising from a general admixture of F-term, D-term, and almost no-scale
supersymmetry breaking, employing non-linear superfields and a novel gauge
fixing for supergravity discussed in a companion paper. In theories of F-term
and D-term breaking, goldstini acquire a mass which is precisely twice the
gravitino mass, while the inclusion of no-scale breaking renders one of these
modes, the modulino, massless. We argue that the vanishing modulino mass can be
explained in terms of an accidental and spontaneously broken "global"
supersymmetry.Comment: 10 pages, 2 figures; v2: typo corrected, references updated; v3:
version to appear in JHE
Flux Pinning and Phase Transitions in Model High-Temperature Superconductors with Columnar Defects
We calculate the degree of flux pinning by defects in model high-temperature
superconductors (HTSC's). The HTSC is modeled as a three-dimensional network of
resistively-shunted Josephson junctions in an external magnetic field,
corresponding to a HTSC in the extreme Type-II limit. Disorder is introduced
either by randomizing the coupling between grains (Model A disorder) or by
removing grains (Model B disorder). Three types of defects are considered:
point disorder, random line disorder, and periodic line disorder; but the
emphasis is on random line disorder. Static and dynamic properties of the
models are determined by Monte Carlo simulations and by solution of the
analogous coupled overdamped Josephson equations in the presence of thermal
noise. Random line defects considerably raise the superconducting transition
temperature T, and increase the apparent critical current density
J, in comparison to the defect-free crystal. They are more effective
in these respects than a comparable volume density of point defects, in
agreement with the experiments of Civale {\it et al}. Periodic line defects
commensurate with the flux lattice are found to raise T even more than
do random line defects. Random line defects are most effective when their
density approximately equals the flux density. Near T, our static and
dynamic results appear consistent with the anisotropic Bose glass scaling
hypotheses of Nelson and Vinokur, but with possibly different critical indices:Comment: 10 pages, LaTeX(REVTeX v3.0, twocolumn), 11 figures (not included
Driven Diffusion in the Two-Dimensional Lattice Coulomb Gas; A Model for Flux Flow in Superconducting Networks
We carry out driven diffusion Monte Carlo simulations of the two dimensional
classical lattice Coulomb gas in an applied uniform electric field, as a model
for vortex motion due to an applied d.c. current, in a periodic superconducting
network. A finite-size version of dynamic scaling is used to extract the
dynamic critical exponent z, and infer the non-linear response at the
transition temperature. We consider the f=0 and f=1/2 cases, corresponding to
no applied magnetic field, and to one half flux quantum per unit cell of the
network respectively.Comment: 25 pages, 7 figures (available from [email protected]),
RevTex3.0, URST12
MR imaging of overuse injuries in the skeletally immature gymnast: spectrum of soft-tissue and osseous lesions in the hand and wrist
In the pediatric gymnast, stress-related physeal injuries have been well described with characteristic imaging findings. However, a spectrum of overuse injuries, some rarely reported in the literature, can be encountered in the gymnast’s hand and wrist.
To demonstrate the MR appearance of a spectrum of overuse injuries in the skeletally immature wrist and hand of pediatric gymnasts.
A total of 125 MR exams of the hand and wrist in skeletally immature children were performed at our institution during a 2-year period. Clinical histories were reviewed for gymnastics participation. MR studies of that subpopulation were reviewed and abnormalities tabulated.
Of the MR studies reviewed, ten gymnasts were identified, all girls age 12–16 years (mean age 14.2 years) who presented with wrist or hand pain. Three of these children had bilateral MR exams. Abnormalities included chronic physeal injuries in three children. Two girls exhibited focal lunate osteochondral defects. Triangular fibrocartilage tears were present in three girls, one of whom had a scapholunate ligament tear. Two girls manifested metacarpal head flattening and necrosis.
A variety of soft-tissue and osseous lesions can be encountered in the skeletally immature gymnast. Familiarity with these stress-related injuries is important for accurate diagnosis
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