234 research outputs found
Small-Angle Neutron Scattering and Magnetization Study of HoNi2B2C
The superconducting and magnetic properties of HoNi2B2C single crystals are
investigated through transport, magnetometry and small-angle neutron scattering
measurements. In the magnetic phases that enter below the superconducting
critical temperature, the small-angle neutron scattering data uncover networks
of magnetic surfaces. These likely originate from uncompensated moments e.g. at
domain walls pinned to crystallographic grain boundaries. The field and
temperature dependent behaviour appears consistent with the metamagnetic
transitions reported in earlier works.Comment: 11 pages , 4 figures, submitted to Low Temperature Physic
Computing Linear Matrix Representations of Helton-Vinnikov Curves
Helton and Vinnikov showed that every rigidly convex curve in the real plane
bounds a spectrahedron. This leads to the computational problem of explicitly
producing a symmetric (positive definite) linear determinantal representation
for a given curve. We study three approaches to this problem: an algebraic
approach via solving polynomial equations, a geometric approach via contact
curves, and an analytic approach via theta functions. These are explained,
compared, and tested experimentally for low degree instances.Comment: 19 pages, 3 figures, minor revisions; Mathematical Methods in
Systems, Optimization and Control, Birkhauser, Base
Behavior of vortices near twin boundaries in underdoped
We use scanning SQUID microscopy to investigate the behavior of vortices in
the presence of twin boundaries in the pnictide superconductor
Ba(Fe1-xCox)2As2. We show that the vortices avoid pinning on twin boundaries.
Individual vortices move in a preferential way when manipulated with the SQUID:
they tend to not cross a twin boundary, but rather to move parallel to it. This
behavior can be explained by the observation of enhanced superfluid density on
twin boundaries in Ba(Fe1-xCox)2As2. The observed repulsion from twin
boundaries may be a mechanism for enhanced critical currents observed in
twinned samples in pnictides and other superconductors
Self-organized criticality induced by quenched disorder: experiments on flux avalanches in NbH films
We present an experimental study of the influence of quenched disorder on the
distribution of flux avalanches in type-II superconductors. In the presence of
much quenched disorder, the avalanche sizes are power-law distributed and show
finite size scaling, as expected from self-organized criticality (SOC).
Furthermore, the shape of the avalanches is observed to be fractal. In the
absence of quenched disorder, a preferred size of avalanches is observed and
avalanches are smooth. These observations indicate that a certain minimum
amount of disorder is necessary for SOC behavior. We relate these findings to
the appearance or non-appearance of SOC in other experimental systems,
particularly piles of sand.Comment: 4 pages, 4 figure
Anomalous f_1 exchange in vector meson photoproduction asymmetries
We perform an analysis of the elastic production of vector mesons with
polarized photon beams at high energy in order to investigate the validity of a
recently proposed dynamical mechanism based on the dominance of the f_1
trajectory at large momentum transfer. The density matrix characterizing the
angular distributions of the vector meson decays is calculated within an
exchange model which includes the Pomeron and the f_1. The asymmetries of these
decays turn out to be very useful to disentangle the role of these exchanges
since their effect depends crucially on their quantum numbers which are
different. The observables analyzed are accessible with present experimental
facilities.Comment: 10 pages, REVTeX, 4 figures, some figures are corrected, conclusions
unchange
A mechanism for the Double-Spin Asymmetry in Electromagnetic Production at HERMES
We calculate the contribution of meson and pomeron exchanges to the
double-spin asymmetry in -meson electromagnetic production at HERMES
energies. We show that the observed double-spin asymmetries, which are large,
can be explained by the interference between the natural parity -secondary
Reggeon and the unnatural parity anomalous exchanges.Comment: 7 pages, 3 figures, Late
Vortex studies in superconducting Ba(Fe0.93Co0.07)2As2
We present small-angle neutron scattering (SANS) and Bitter decoration
studies of the superconducting vortices in
Ba(FeCo)As}. A highly disordered vortex configuration
is observed at all measured fields, and is attributed to strong pinning. This
conclusion is supported by the absence of a Meissner rim in decoration images
obtained close to the sample edge. The field dependence of the magnitude of the
SANS scattering vector indicates vortex lattice domains of (distorted)
hexagonal symmetry, consistent with the decoration images which show primarily
6-fold coordinated vortex domains. An analysis of the scattered intensity shows
that this decreases much more rapidly than expected from estimates of the upper
critical field, consistent with the large degree of disorder.Comment: 5 pages, 4 figure
On the class SI of J-contractive functions intertwining solutions of linear differential equations
In the PhD thesis of the second author under the supervision of the third
author was defined the class SI of J-contractive functions, depending on a
parameter and arising as transfer functions of overdetermined conservative 2D
systems invariant in one direction. In this paper we extend and solve in the
class SI, a number of problems originally set for the class SC of functions
contractive in the open right-half plane, and unitary on the imaginary line
with respect to some preassigned signature matrix J. The problems we consider
include the Schur algorithm, the partial realization problem and the
Nevanlinna-Pick interpolation problem. The arguments rely on a correspondence
between elements in a given subclass of SI and elements in SC. Another
important tool in the arguments is a new result pertaining to the classical
tangential Schur algorithm.Comment: 46 page
Effect of electron irradiation on vortex dynamics in YBa_2Cu_3O_{7-x} single crystals
We report on drastic change of vortex dynamics with increase of quenched
disorder: for rather weak disorder we found a single vortex creep regime, which
we attribute to a Bragg-glass phase, while for enhanced disorder we found an
increase of both the depinning current and activation energy with magnetic
field, which we attribute to entangled vortex phase. We also found that
introduction of additional defects always increases the depinning current, but
it increases activation energy only for elastic vortex creep, while it
decreases activation energy for plastic vortex creep.Comment: 4 pages, 3 figures, submited to Phys. Rev.
GCIP water and energy budget synthesis (WEBS)
As part of the World Climate Research Program\u27s (WCRPs) Global Energy and Water-Cycle Experiment (GEWEX) Continental-scale International Project (GCIP), a preliminary water and energy budget synthesis (WEBS) was developed for the period 1996–1999 from the “best available” observations and models. Besides this summary paper, a companion CD-ROM with more extensive discussion, figures, tables, and raw data is available to the interested researcher from the GEWEX project office, the GAPP project office, or the first author. An updated online version of the CD-ROM is also available at http://ecpc.ucsd.edu/gcip/webs.htm/. Observations cannot adequately characterize or “close” budgets since too many fundamental processes are missing. Models that properly represent the many complicated atmospheric and near-surface interactions are also required. This preliminary synthesis therefore included a representative global general circulation model, regional climate model, and a macroscale hydrologic model as well as a global reanalysis and a regional analysis. By the qualitative agreement among the models and available observations, it did appear that we now qualitatively understand water and energy budgets of the Mississippi River Basin. However, there is still much quantitative uncertainty. In that regard, there did appear to be a clear advantage to using a regional analysis over a global analysis or a regional simulation over a global simulation to describe the Mississippi River Basin water and energy budgets. There also appeared to be some advantage to using a macroscale hydrologic model for at least the surface water budgets
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