747 research outputs found
Composite Topological Excitations in Ferromagnet-Superconductor Heterostructures
We investigate the formation of a new type of composite topological
excitation -- the skyrmion-vortex pair (SVP) -- in hybrid systems consisting of
coupled ferromagnetic and superconducting layers. Spin-orbit interaction in the
superconductor mediates a magnetoelectric coupling between the vortex and the
skyrmion, with a sign (attractive or repulsive) that depends on the topological
indices of the constituents. We determine the conditions under which a bound
SVP is formed, and characterize the range and depth of the effective binding
potential through analytical estimates and numerical simulations. Furthermore,
we develop a semiclassical description of the coupled skyrmion-vortex dynamics
and discuss how SVPs can be controlled by applied spin currents.Comment: Final version accepted by Physical Review Letters; 9 pages, 5 figure
Critical velocity of a mobile impurity in one-dimensional quantum liquids
We study the notion of superfluid critical velocity in one spatial dimension.
It is shown that for heavy impurities with mass exceeding a critical mass
, the dispersion develops periodic metastable branches resulting
in dramatic changes of dynamics in the presence of an external driving force.
In contrast to smooth Bloch Oscillations for , a heavy impurity
climbs metastable branches until it reaches a branch termination point or
undergoes a random tunneling event, both leading to an abrupt change in
velocity and an energy loss. This is predicted to lead to a non-analytic
dependence of the impurity drift velocity on small forces.Comment: 5 pages, 2 figures; New version with Supplemental Material (3 pages,
6 figures); Accepted to PR
University-community engagement: The Fresno story of targeted neighborhood revitalization
In this article we take a closer look at a developing university-community engagement project being undertaken between California State University, Fresno, and the City of Fresno. A history of the project is provided, along with a review of the relevant literature and a summary of what pieces of the puzzle we feel should be in place for a successful collaboration of this sort. These include what structures should be institutionalized for successful collaboration at the university, in the partnering organizations, and in the community
Neutral Plasma Oscillations at Zero Temperature
We use cold plasma theory to calculate the response of an ultracold neutral
plasma to an applied rf field. The free oscillation of the system has a
continuous spectrum and an associated damped quasimode. We show that this
quasimode dominates the driven response. We use this model to simulate plasma
oscillations in an expanding ultracold neutral plasma, providing insights into
the assumptions used to interpret experimental data [Phys. Rev. Lett. 85, 318
(2000)].Comment: 4.3 pages, including 3 figure
Numerical Simulation of Vortex Crystals and Merging in N-Point Vortex Systems with Circular Boundary
In two-dimensional (2D) inviscid incompressible flow, low background
vorticity distribution accelerates intense vortices (clumps) to merge each
other and to array in the symmetric pattern which is called ``vortex
crystals''; they are observed in the experiments on pure electron plasma and
the simulations of Euler fluid. Vortex merger is thought to be a result of
negative ``temperature'' introduced by L. Onsager. Slight difference in the
initial distribution from this leads to ``vortex crystals''. We study these
phenomena by examining N-point vortex systems governed by the Hamilton
equations of motion. First, we study a three-point vortex system without
background distribution. It is known that a N-point vortex system with boundary
exhibits chaotic behavior for N\geq 3. In order to investigate the properties
of the phase space structure of this three-point vortex system with circular
boundary, we examine the Poincar\'e plot of this system. Then we show that
topology of the Poincar\'e plot of this system drastically changes when the
parameters, which are concerned with the sign of ``temperature'', are varied.
Next, we introduce a formula for energy spectrum of a N-point vortex system
with circular boundary. Further, carrying out numerical computation, we
reproduce a vortex crystal and a vortex merger in a few hundred point vortices
system. We confirm that the energy of vortices is transferred from the clumps
to the background in the course of vortex crystallization. In the vortex
merging process, we numerically calculate the energy spectrum introduced above
and confirm that it behaves as k^{-\alpha},(\alpha\approx 2.2-2.8) at the
region 10^0<k<10^1 after the merging.Comment: 30 pages, 11 figures. to be published in Journal of Physical Society
of Japan Vol.74 No.
Three-dimensional coherent X-ray diffraction imaging of a ceramic nanofoam: determination of structural deformation mechanisms
Ultra-low density polymers, metals, and ceramic nanofoams are valued for
their high strength-to-weight ratio, high surface area and insulating
properties ascribed to their structural geometry. We obtain the labrynthine
internal structure of a tantalum oxide nanofoam by X-ray diffractive imaging.
Finite element analysis from the structure reveals mechanical properties
consistent with bulk samples and with a diffusion limited cluster aggregation
model, while excess mass on the nodes discounts the dangling fragments
hypothesis of percolation theory.Comment: 8 pages, 5 figures, 30 reference
Elevated osteoprotegerin is associated with abnormal ankle brachial indices in patients infected with HIV: a cross-sectional study
<p>Abstract</p> <p>Background</p> <p>Patients infected with HIV have an increased risk for accelerated atherosclerosis. Elevated levels of osteoprotegerin, an inflammatory cytokine receptor, have been associated with a high incidence of cardiovascular disease (including peripheral arterial disease, or PAD), acute coronary syndrome, and cardiovascular mortality. The objective of this study was to determine whether PAD is prevalent in an HIV-infected population, and to identify an association with HIV-specific and traditional cardiovascular risk factors, as well as levels of osteoprotegerin.</p> <p>Methods</p> <p>One hundred and two patients infected with HIV were recruited in a cross-sectional study. To identify the prevalence of PAD, ankle-brachial indices (ABIs) were measured. Four standard ABI categories were utilized: ≤ 0.90 (definite PAD); 0.91-0.99 (borderline); 1.00-1.30 (normal); and >1.30 (high). Medical history and laboratory measurements were obtained to determine possible risk factors associated with PAD in HIV-infected patients.</p> <p>Results</p> <p>The prevalence of PAD (ABI ≤ 0.90) in a young HIV-infected population (mean age: 48 years) was 11%. Traditional cardiovascular risk factors, including advanced age and previous cardiovascular history, as well as elevated C-reactive protein levels, were associated with PAD. Compared with patients with normal ABIs, patients with high ABIs had significantly elevated levels of osteoprotegerin [1428.9 (713.1) pg/ml vs. 3088.6 (3565.9) pg/ml, respectively, p = 0.03].</p> <p>Conclusions</p> <p>There is a high prevalence of PAD in young HIV-infected patients. A number of traditional cardiovascular risk factors and increased osteoprotegerin concentrations are associated with abnormal ABIs. Thus, early screening and aggressive medical management for PAD may be warranted in HIV-infected patients.</p
Quasi-stationary States of Two-Dimensional Electron Plasma Trapped in Magnetic Field
We have performed numerical simulations on a pure electron plasma system
under a strong magnetic field, in order to examine quasi-stationary states that
the system eventually evolves into. We use ring states as the initial states,
changing the width, and find that the system evolves into a vortex crystal
state from a thinner-ring state while a state with a single-peaked density
distribution is obtained from a thicker-ring initial state. For those
quasi-stationary states, density distribution and macroscopic observables are
defined on the basis of a coarse-grained density field. We compare our results
with experiments and some statistical theories, which include the
Gibbs-Boltzmann statistics, Tsallis statistics, the fluid entropy theory, and
the minimum enstrophy state. From some of those initial states, we obtain the
quasi-stationary states which are close to the minimum enstrophy state, but we
also find that the quasi-stationary states depend upon initial states, even if
the initial states have the same energy and angular momentum, which means the
ergodicity does not hold.Comment: 9 pages, 7 figure
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