573 research outputs found
Numerical study of vortex system quantum melting
We report a numerical study of the vortex system in the two dimensional
II-type superconductors. We have proposed a phenomenological model that takes
into account quantum fluctuations of Abrikosov's vortices. The results of the
quantum Monte-Carlo simulations by the SSE algorithm show that the thermal
fluctuations are dominated by quantum fluctuations at low temperatures. In
particular, we demonstrate the possibility of the quantum melting transition
for vortex system in the temperature region where thermal melting transition is
improbable
On the reconstruction of planar lattice-convex sets from the covariogram
A finite subset of is said to be lattice-convex if is
the intersection of with a convex set. The covariogram of
is the function associating to each u \in
\integer^d the cardinality of . Daurat, G\'erard, and Nivat and
independently Gardner, Gronchi, and Zong raised the problem on the
reconstruction of lattice-convex sets from . We provide a partial
positive answer to this problem by showing that for and under mild extra
assumptions, determines up to translations and reflections. As a
complement to the theorem on reconstruction we also extend the known
counterexamples (i.e., planar lattice-convex sets which are not
reconstructible, up to translations and reflections) to an infinite family of
counterexamples.Comment: accepted in Discrete and Computational Geometr
Topological Defects in Gravitational Lensing Shear Fields
Shear fields due to weak gravitational lensing have characteristic coherent
patterns. We describe the topological defects in shear fields in terms of the
curvature of the surface described by the lensing potential. A simple
interpretation of the characteristic defects is given in terms of the the
umbilical points of the potential surface produced by ellipsoidal halos. We
show simulated lensing shear maps and point out the typical defect
configurations. Finally, we show how statistical properties such as the
abundance of defects can be expressed in terms of the correlation function of
the lensing potential.Comment: 17 pages, 4 figure
Fouling in Nanofiltration
According to Koros et al. [1] fouling is “the process resulting in loss of performance of a membrane due to deposition
of suspended or dissolved substances on its external surfaces, at its pore openings, or within its pores”. Fouling is also
decribed as flux decline which is irreversible and can only be removed by, for example, chemical
cleaning [2]. This is different to flux decline due to solution chemistry effects or concentration
polarisation which is described in more detail later in this chapter. Those flux declines can be reversed
with clean water and are hence not considered as fouling
Critical-point scaling function for the specific heat of a Ginzburg-Landau superconductor
If the zero-field transition in high temperature superconductors such as
YBa_2Cu_3O_7-\delta is a critical point in the universality class of the
3-dimensional XY model, then the general theory of critical phenomena predicts
the existence of a critical region in which thermodynamic functions have a
characteristic scaling form. We report the first attempt to calculate the
universal scaling function associated with the specific heat, for which
experimental data have become available in recent years. Scaling behaviour is
extracted from a renormalization-group analysis, and the 1/N expansion is
adopted as a means of approximation. The estimated scaling function is
qualitatively similar to that observed experimentally, and also to the
lowest-Landau-level scaling function used by some authors to provide an
alternative interpretation of the same data. Unfortunately, the 1/N expansion
is not sufficiently reliable at small values of N for a quantitative fit to be
feasible.Comment: 20 pages; 4 figure
Microlensing by natural wormholes: theory and simulations
We provide an in depth study of the theoretical peculiarities that arise in
effective negative mass lensing, both for the case of a point mass lens and
source, and for extended source situations. We describe novel observational
signatures arising in the case of a source lensed by a negative mass. We show
that a negative mass lens produces total or partial eclipse of the source in
the umbra region and also show that the usual Shapiro time delay is replaced
with an equivalent time gain. We describe these features both theoretically, as
well as through numerical simulations. We provide negative mass microlensing
simulations for various intensity profiles and discuss the differences between
them. The light curves for microlensing events are presented and contrasted
with those due to lensing produced by normal matter. Presence or absence of
these features in the observed microlensing events can shed light on the
existence of natural wormholes in the Universe.Comment: 16 pages, 24 postscript figures (3 coloured), revtex style, submitted
to Phys. Rev.
Three-body vs. dineutron approach to two-neutron radiative capture in He
The low-energy behavior of the strength function for the soft dipole
excitation in He is studied theoretically. Use of very large basis sizes
and well-grounded extrapolation procedures allows to move to energies as small
as 1 keV, at which the low-energy asymptotic behavior of the E1 strength
function seems to be achieved. It is found that the low-energy behavior of the
strength function is well described in the effective three-body "dynamical
dineutron model". The astrophysical rate for the ++He+ is calculated. Comparison with the previous calculations is
performed.Comment: 6 pages, 9 figure
NMR and NQR Fluctuation Effects in Layered Superconductors
We study the effect of thermal fluctuations of the s-wave order parameter of
a quasi two dimensional superconductor on the nuclear spin relaxation rate near
the transition temperature Tc. We consider both the effects of the amplitude
fluctuations and the Berezinskii-Kosterlitz-Thouless (BKT) phase fluctuations
in weakly coupled layered superconductors. In the treatment of the amplitude
fluctuations we employ the Gaussian approximation and evaluate the longitudinal
relaxation rate 1/T1 for a clean s-wave superconductor, with and without pair
breaking effects, using the static pair fluctuation propagator D. The increase
in 1/T1 due to pair breaking in D is overcompensated by the decrease arising
from the single particle Green's functions. The result is a strong effect on
1/T1 for even a small amount of pair breaking. The phase fluctuations are
described in terms of dynamical BKT excitations in the form of pancake
vortex-antivortex (VA) pairs. We calculate the effect of the magnetic field
fluctuations caused by the translational motion of VA excitations on 1/T1 and
on the transverse relaxation rate 1/T2 on both sides of the BKT transitation
temperature T(BKT)<Tc. The results for the NQR relaxation rates depend strongly
on the diffusion constant that governs the motion of free and bound vortices as
well as the annihilation of VA pairs. We discuss the relaxation rates for real
multilayer systems where the diffusion constant can be small and thus increase
the lifetime of a VA pair, leading to an enhancement of the rates. We also
discuss in some detail the experimental feasibility of observing the effects of
amplitude fluctuations in layered s-wave superconductors such as the
dichalcogenides and the effects of phase fluctuations in s- or d-wave
superconductors such as the layered cuprates.Comment: 38 pages, 12 figure
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