44,150 research outputs found
--Oscillations for Correlated Electron Pairs in Disordered Mesoscopic Rings
The full spectrum of two interacting electrons in a disordered mesoscopic
one--dimensional ring threaded by a magnetic flux is calculated numerically.
For ring sizes far exceeding the one--particle localization length we
find several --periodic states whose eigenfunctions exhibit a pairing
effect. This represents the first direct observation of interaction--assisted
coherent pair propagation, the pair being delocalized on the scale of the whole
ring.Comment: 4 pages, uuencoded PostScript, containing 5 figures
A new stellar mixing process operating below shell convection zones following off-center ignition
During most stages of stellar evolution the nuclear burning of lighter to
heavier elements results in a radial composition profile which is stabilizing
against buoyant acceleration, with light material residing above heavier
material. However, under some circumstances, such as off-center ignition, the
composition profile resulting from nuclear burning can be destabilizing, and
characterized by an outwardly increasing mean molecular weight. The potential
for instabilities under these circumstances, and the consequences that they may
have on stellar structural evolution, remain largely unexplored. In this paper
we study the development and evolution of instabilities associated with
unstable composition gradients in regions which are initially stable according
to linear Schwarzschild and Ledoux criteria. In particular, we explore the
mixing taking place under various conditions with multi-dimensional
hydrodynamic convection models based on stellar evolutionary calculations of
the core helium flash in a 1.25 \Msun star, the core carbon flash in a
9.3\,\Msun star, and of oxygen shell burning in a star with a mass of
23\,\Msun. The results of our simulations reveal a mixing process associated
with regions having outwardly increasing mean molecular weight that reside
below convection zones. The mixing is not due to overshooting from the
convection zone, nor is it due directly to thermohaline mixing which operates
on a timescale several orders of magnitude larger than the simulated flows.
Instead, the mixing appears to be due to the presence of a wave field induced
in the stable layers residing beneath the convection zone which enhances the
mixing rate by many orders of magnitude and allows a thermohaline type mixing
process to operate on a dynamical, rather than thermal, timescale. We discuss
our results in terms of related laboratory phenomena and associated theoretical
developments.Comment: accepted for publication in Astrophysical Journal, 9 pages, 8 figure
A model for the phase separation controlled by doping and the internal chemical pressure in different cuprate superconductors
In the framework of a two-band model, we study the phase separation regime of
different kinds of strongly correlated charge carriers as a function of the
energy splitting between the two sets of bands. The narrow (wide) band
simulates the more localized (more delocalized) type of charge carriers. By
assuming that the internal chemical pressure on the CuO layer due to
interlayer mismatch controls the energy splitting between the two sets of
states, the theoretical predictions are able to reproduce the regime of phase
separation at doping higher than 1/8 in the experimental pressure-doping-
phase diagram of cuprates at large microstrain as it appears in overoxygenated
LaCuO.Comment: 8 pages, 5 figures, submitted to Phys. Rev.
A simple model of price formation
A simple Ising spin model which can describe the mechanism of price formation
in financial markets is proposed. In contrast to other agent-based models, the
influence does not flow inward from the surrounding neighbors to the center
site, but spreads outward from the center to the neighbors. The model thus
describes the spread of opinions among traders. It is shown via standard Monte
Carlo simulations that very simple rules lead to dynamics that duplicate those
of asset prices.Comment: Version 2: 4 pages, 4 figures; added more stringent statistical
analysis; to appear in Int. J. Modern Physics C, Vol. 13, No. 1 (2002
Lattice study of the infrared behavior of QCD Green's functions in Landau gauge
We summarize the current status of our numerical results for the gluon and
ghost propagators and for the Kugo-Ojima confinement parameter in quenched
SU(3) lattice Landau gauge theory. The data for the propagators are compared to
our results obtained in the case of full QCD, simulated using two flavors of
dynamical clover-improved Wilson fermions. We demonstrate that the infrared
behavior of the ghost propagator is consistent with the Kugo-Ojima confinement
criterion. Explicit violation of reflection positivity by the gluon propagator
is shown. Additionally, we present results of a running coupling constant both
at low and at large momenta.Comment: 7 pages, 8 figures, talk presented at Lattice2006 (Confinement and
Topology
Landau Gauge Gluon and Ghost Propagators from Lattice QCD
We report on recent numerical computations of the Landau gauge gluon and
ghost propagators as well as of the ghost-gluon vertex function in pure SU(3)
Yang-Mills theory and in full QCD on the lattice. Special emphasis is paid to
the low momentum region. In particular, we present new data for the gluon
propagator at momenta below 300 MeV. We also discuss different systematic
effects as there are finite-size, lattice discretization and Gribov copy but
also unquenching effects. A MOM-scheme running coupling \alpha_s(q^2) based on
the ghost-gluon vertex is calculated and found to decrease for momenta below
550 MeV, even though the renormalization constant of the vertex deviates only
weakly from being constant.Comment: Talk at the Workshop IRQCD '06, Rio de Janeiro, June 2006, given by
M. M{\"u}ller-Preussker; 8 pages, 9 figures, a few wordings made more
precise, figures put into equal style, missing legends adde
Deeply virtual Compton scattering in next-to-leading order
We study the amplitude of deeply virtual Compton scattering in
next-to-leading order of perturbation theory including the two-loop evolution
effects for different sets of skewed parton distributions (SPDs). It turns out
that in the minimal subtraction scheme the relative radiative corrections are
of order 20-50%. We analyze the dependence of our predictions on the choice of
SPD, that will allow to discriminate between possible models of SPDs from
future high precision experimental data, and discuss shortly theoretical
uncertainties induced by the radiative corrections.Comment: 10 pages, LaTeX, 3 figure
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