10,608 research outputs found
Vortex lattice melting in layered superconductors with periodic columnar pins
The melting transition of the vortex lattice in highly anisotropic, layered
superconductors with commensurate, periodic columnar pins is studied in a
geometry where magnetic field and columnar pins are normal to the layers.
Thermodynamic properties and equilibrium density distributions are obtained
from numerical minimizations of an appropriate free-energy functional. We find
a line of first-order transitions that ends at a critical point as the pin
concentration is increased. A simple Landau theory providing a
semi-quantitative explanation of the numerical results is proposed.Comment: Four pages, 3 Figure
Comparative study between two quantum spin systems KCuCl and TlCuCl
We have performed an {\it ab initio} study of the electronic structure of two
isostructural quantum spin systems, KCuCl and TlCuCl, which have
recently attracted much attention due to their unconventional magnetic
properties. Our first-principles analysis shows unambiguously the role of Tl,
as opposed to structural differences between the two compounds, in making
TlCuCl a {\it strongly} coupled s=1/2 dimer system compared to
KCuCl which shows a {\it weakly} coupled s=1/2 dimer behavior. Good
agreement with the existing analysis of inelastic neutron scattering results
has been observed.Comment: minor changes in text with additional references; in EPL forma
Non-classical Rotational Inertia in a Two-dimensional Bosonic Solid Containing Grain Boundaries
We study the occurrence of non-classical rotational inertia (NCRI) arising
from superfluidity along grain boundaries in a two-dimensional bosonic system.
We make use of a standard mapping between the zero-temperature properties of
this system and the statistical mechanics of interacting vortex lines in the
mixed phase of a type-II superconductor. In the mapping, the liquid phase of
the vortex system corresponds to the superfluid bosonic phase. We consider
numerically obtained polycrystalline configurations of the vortex lines in
which the microcrystals are separated by liquid-like grain boundary regions
which widen as the vortex system temperature increases. The NCRI of the
corresponding zero-temperature bosonic systems can then be numerically
evaluated by solving the equations of superfluid hydrodynamics in the channels
near the grain boundaries. We find that the NCRI increases very abruptly as the
liquid regions in the vortex system (equivalently, superfluid regions in the
bosonic system) form a connected, system-spannig structure with one or more
closed loops. The implications of these results for experimentally observed
supersolid phenomena are discussed.Comment: Ten pages, including figure
Phase diagram of randomly pinned vortex matter in layered superconductors: dependence on the details of the point pinning
We study the thermodynamic and structural properties of the superconducting
vortex system in high temperature layered superconductors, with magnetic field
normal to the layers, in the presence of a small concentration of strong random
point pinning defects via numerical minimization of a model free energy
functional in terms of the time-averaged local density of pancake vortices.
Working at constant magnetic induction and point pinning center concentration,
we find that the equilibrium phase at low temperature () and small pinning
strength () is a topologically ordered Bragg glass. As or is
increased, the Bragg glass undergoes a first order transition to a disordered
phase which we characterize as a ``vortex slush'' with polycrystalline
structure within the layers and interlayer correlations extending to about
twenty layers. This is in contrast with the pinned vortex liquid phase into
which the Bragg glass was found to melt, using the same methods, in the case of
a large concentration of weak pinning centers: that phase was amorphous with
very little interlayer correlation. The value of the second moment of the
random pinning potential at which the Bragg glass melts for a fixed temperature
is very different in the two systems. These results imply that the effects of
random point pinning can not be described only in terms of the second moment of
the pinning potential, and that some of the unresolved contradictions in the
literature concerning the nature of the low and high phase in this
system are likely to arise from differences in the nature of the pinning in
different samples, or from assumptions made about the pinning potential.Comment: 13 pages including 11 figures. Typos in HTML abstract corrected in v
Electronic and magnetic structure of CsVO
We have studied the electronic structure of the spin-gapped system
CsVO by means of an ab initio calculation.
Our analysis and a re-examination of the susceptibility data indicate that
the behavior of this system is much closer to that of an alternating spin-1/2
antiferromagnetic chain with significant inter-dimer coupling and weaker
inter-chain couplings than that of isolated dimers as was initially proposed.
Comparison to the vanadate family members -NaVO,
-LiVO and isostructural compounds like
(VO)PO (VOPO) is discussed.Comment: revised version. To appear in Phys. Rev.
Order independent structural alignment of circularly permuted proteins
Circular permutation connects the N and C termini of a protein and
concurrently cleaves elsewhere in the chain, providing an important mechanism
for generating novel protein fold and functions. However, their in genomes is
unknown because current detection methods can miss many occurances, mistaking
random repeats as circular permutation. Here we develop a method for detecting
circularly permuted proteins from structural comparison. Sequence order
independent alignment of protein structures can be regarded as a special case
of the maximum-weight independent set problem, which is known to be
computationally hard. We develop an efficient approximation algorithm by
repeatedly solving relaxations of an appropriate intermediate integer
programming formulation, we show that the approximation ratio is much better
then the theoretical worst case ratio of . Circularly permuted
proteins reported in literature can be identified rapidly with our method,
while they escape the detection by publicly available servers for structural
alignment.Comment: 5 pages, 3 figures, Accepted by IEEE-EMBS 2004 Conference Proceeding
Entropic Origin of the Growth of Relaxation Times in Simple Glassy Liquids
Transitions between ``glassy'' local minima of a model free-energy functional
for a dense hard-sphere system are studied numerically using a
``microcanonical'' Monte Carlo method that enables us to obtain the transition
probability as a function of the free energy and the Monte Carlo ``time''. The
growth of the height of the effective free energy barrier with density is found
to be consistent with a Vogel-Fulcher law. The dependence of the transition
probability on time indicates that this growth is primarily due to entropic
effects arising from the difficulty of finding low-free-energy saddle points
connecting glassy minima.Comment: Four pages, plus three postscript figure
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