7,927 research outputs found
Three-body properties of low-lying Be resonances
We compute the three-body structure of the lowest resonances of Be
considered as two neutrons around an inert Be core. This is an extension
of the bound state calculations of Be into the continuum spectrum. We
investigate the lowest resonances of angular momenta and parities, ,
and . Surprisingly enough, they all are naturally occurring in
the three-body model. We calculate bulk structure dominated by small distance
properties as well as decays determined by the asymptotic large-distance
structure. Both and have two-body Be-neutron d-wave
structure, while has an even mixture of and d-waves. The
corresponding relative neutron-neutron partial waves are distributed among ,
, and d-waves. The branching ratios show different mixtures of one-neutron
emission, three-body direct, and sequential decays. We argue for spin and
parities, , and , to the resonances at 0.89, 2.03, 5.13,
respectively. The computed structures are in agreement with existing reaction
measurements.Comment: To be published in Physical Review
Stochastics theory of log-periodic patterns
We introduce an analytical model based on birth-death clustering processes to
help understanding the empirical log-periodic corrections to power-law scaling
and the finite-time singularity as reported in several domains including
rupture, earthquakes, world population and financial systems. In our
stochastics theory log-periodicities are a consequence of transient clusters
induced by an entropy-like term that may reflect the amount of cooperative
information carried by the state of a large system of different species. The
clustering completion rates for the system are assumed to be given by a simple
linear death process. The singularity at t_{o} is derived in terms of
birth-death clustering coefficients.Comment: LaTeX, 1 ps figure - To appear J. Phys. A: Math & Ge
Dendritic flux patterns in MgB2 films
Magneto-opitcal studies of a c-oriented epitaxial MgB2 film with critical
current density 10^7 A/cm^2 demonstrate a breakdown of the critical state at
temperatures below 10 K [cond-mat/0104113]. Instead of conventional uniform and
gradual flux penetration in an applied magnetic field, we observe an abrupt
invasion of complex dendritic structures. When the applied field subsequently
decreases, similar dendritic structures of the return flux penetrate the film.
The static and dynamic properties of the dendrites are discussed.Comment: Accepted to Supercond. Sci. Techno
Dynamic filtering of static dipoles in magnetoencephalography
We consider the problem of estimating neural activity from measurements
of the magnetic fields recorded by magnetoencephalography. We exploit
the temporal structure of the problem and model the neural current as a
collection of evolving current dipoles, which appear and disappear, but whose
locations are constant throughout their lifetime. This fully reflects the physiological
interpretation of the model.
In order to conduct inference under this proposed model, it was necessary
to develop an algorithm based around state-of-the-art sequential Monte
Carlo methods employing carefully designed importance distributions. Previous
work employed a bootstrap filter and an artificial dynamic structure
where dipoles performed a random walk in space, yielding nonphysical artefacts
in the reconstructions; such artefacts are not observed when using the
proposed model. The algorithm is validated with simulated data, in which
it provided an average localisation error which is approximately half that of
the bootstrap filter. An application to complex real data derived from a somatosensory
experiment is presented. Assessment of model fit via marginal
likelihood showed a clear preference for the proposed model and the associated
reconstructions show better localisation
Quantum theory of successive projective measurements
We show that a quantum state may be represented as the sum of a joint
probability and a complex quantum modification term. The joint probability and
the modification term can both be observed in successive projective
measurements. The complex modification term is a measure of measurement
disturbance. A selective phase rotation is needed to obtain the imaginary part.
This leads to a complex quasiprobability, the Kirkwood distribution. We show
that the Kirkwood distribution contains full information about the state if the
two observables are maximal and complementary. The Kirkwood distribution gives
a new picture of state reduction. In a nonselective measurement, the
modification term vanishes. A selective measurement leads to a quantum state as
a nonnegative conditional probability. We demonstrate the special significance
of the Schwinger basis.Comment: 6 page
Mechanism for flux guidance by micrometric antidot arrays in superconducting films
A study of magnetic flux penetration in a superconducting film patterned with
arrays of micron sized antidots (microholes) is reported. Magneto-optical
imaging (MOI) of a YBCO film shaped as a long strip with perpendicular antidot
arrays revealed both strong guidance of flux, and at the same time large
perturbations of the overall flux penetration and flow of current. These
results are compared with a numerical flux creep simulation of a thin
superconductor with the same antidot pattern. To perform calculations on such a
complex geometry, an efficient numerical scheme for handling the boundary
conditions of the antidots and the nonlocal electrodynamics was developed. The
simulations reproduce essentially all features of the MOI results. In addition,
the numerical results give insight into all other key quantities, e.g., the
electrical field, which becomes extremely large in the narrow channels
connecting the antidots.Comment: 8 pages, 7 figure
Current-induced dendritic magnetic instability in superconducting MgB2 films
Magneto-optical imaging reveals that in superconducting films of MgB2 a
transport current creates avalanche-like flux dynamics where highly branching
dendritic penetration patterns are formed. The instability is triggered when
the current exceeds a threshold value, and the superconductor, shaped as a long
strip, is initially in the critical state. The instability exists up to 19 K,
which is a much wider temperature range than in previous experiments, where
dendrites were formed by applying a magnetic field. The instability is believed
to be of thermo-magnetic origin indicating that thermal stabilization may
become crucial in applications of MgB2.Comment: 3 pages, 3 figures, resubmitted to Appl.Phys.Let
Synchronization and Coarsening (without SOC) in a Forest-Fire Model
We study the long-time dynamics of a forest-fire model with deterministic
tree growth and instantaneous burning of entire forests by stochastic lightning
strikes. Asymptotically the system organizes into a coarsening self-similar
mosaic of synchronized patches within which trees regrow and burn
simultaneously. We show that the average patch length grows linearly with
time as t-->oo. The number density of patches of length L, N(L,t), scales as
^{-2}M(L/), and within a mean-field rate equation description we find
that this scaling function decays as e^{-1/x} for x-->0, and as e^{-x} for
x-->oo. In one dimension, we develop an event-driven cluster algorithm to study
the asymptotic behavior of large systems. Our numerical results are consistent
with mean-field predictions for patch coarsening.Comment: 5 pages, 4 figures, 2-column revtex format. To be submitted to PR
Superconductor strip with transport current: Magneto-optical study of current distribution and its relaxation
The dynamics of magnetic flux distributions across a YBaCuO strip carrying
transport current is measured using magneto-optical imaging at 20 K. The
current is applied in pulses of 40-5000 ms duration and magnitude close to the
critical one, 5.5 A. During the pulse some extra flux usually penetrates the
strip, so the local field increases in magnitude. When the strip is initially
penetrated by flux, the local field either increases or decreases depending
both on the spatial coordinate and the current magnitude. Meanwhile, the
current density always tends to redistribute more uniformly. Despite the
relaxation, all distributions remain qualitatively similar to the Bean model
predictions.Comment: RevTeX, 9 pages, 9 figures, submitted to Supercond. Sci. Technol.
Revision: MO image and more refs are adde
Topological field theory and physics
Topological Yang-Mills theory with the Belavin-Polyakov-Schwarz-Tyupkin
instanton is solved completely, revealing an underlying multi-link
intersection theory. Link invariants are also shown to survive the coupling to
a certain kind of matter (hyperinstantons). The physical relevance of
topological field theory and its invariants is discovered. By embedding
topological Yang-Mills theory into pure Yang-Mills theory, it is shown that the
topological version TQFT of a quantum field theory QFT allows us to formulate
consistently the perturbative expansion of QFT in the topologically nontrivial
sectors. In particular, TQFT classifies the set of good measures over the
instanton moduli space and solves the inconsistency problems of the previous
approaches. The qualitatively new physical implications are pointed out. Link
numbers in QCD are related to a non abelian analogoue of the Aharonov-Bohm
effect.Comment: 23 pages, 1 figure. Revision: additional explanation
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