1,481 research outputs found
Hydrodynamics of liquids of arbitrarily curved flux-lines and vortex loops
We derive a hydrodynamic model for a liquid of arbitrarily curved flux-lines
and vortex loops using the mapping of the vortex liquid onto a liquid of
relativistic charged quantum bosons in 2+1 dimensions recently suggested by
Tesanovic and by Sudbo and collaborators. The loops in the flux-line system
correspond to particle-antiparticle fluctuations in the bosons. We explicitly
incorporate the externally applied magnetic field which in the boson model
corresponds to a chemical potential associated with the conserved charge
density of the bosons. We propose this model as a convenient and physically
appealing starting point for studying the properties of the vortex liquid
Flux-line entanglement as the mechanism of melting transition in high-temperature superconductors in a magnetic field
The mechanism of the flux-line-lattice (FLL) melting in anisotropic high-T_c
superconductors in is clarified by Monte Carlo
simulations of the 3D frustrated XY model. The percentage of entangled flux
lines abruptly changes at the melting temperature T_m, while no sharp change
can be found in the number and size distribution of vortex loops around T_m.
Therefore, the origin of this melting transition is the entanglement of flux
lines. Scaling behaviors of physical quantities are consistent with the above
mechanism of the FLL melting. The Lindemann number is also evaluated without
any phenomenological arguments.Comment: 10 pages, 5 Postscript figures, RevTeX; changed content and figures,
Phys. Rev. B Rapid Commun. in pres
Reorientation of magnetic anisotropy in epitaxial cobalt ferrite thin films
Spin reorientation has been observed in CoFe2O4 thin single crystalline films epitaxially grown on (100) MgO substrate upon varying the film thickness. The critical thickness for such a spin-reorientation transition was estimated to be 300 nm. The reorientation is driven by a structural transition in the film from a tetragonal to cubic symmetry. At low thickness, the in-plane tensile stress induces a tetragonal distortion of the lattice that generates a perpendicular anisotropy, large enough to overcome the shape anisotropy and to stabilize the magnetization easy axis out of plane. However, in thicker films, the lattice relaxation toward the cubic structure of the bulk allows the shape anisotropy to force the magnetization to be in plane aligned
Gradual sub-lattice reduction and a new complexity for factoring polynomials
We present a lattice algorithm specifically designed for some classical
applications of lattice reduction. The applications are for lattice bases with
a generalized knapsack-type structure, where the target vectors are boundably
short. For such applications, the complexity of the algorithm improves
traditional lattice reduction by replacing some dependence on the bit-length of
the input vectors by some dependence on the bound for the output vectors. If
the bit-length of the target vectors is unrelated to the bit-length of the
input, then our algorithm is only linear in the bit-length of the input
entries, which is an improvement over the quadratic complexity floating-point
LLL algorithms. To illustrate the usefulness of this algorithm we show that a
direct application to factoring univariate polynomials over the integers leads
to the first complexity bound improvement since 1984. A second application is
algebraic number reconstruction, where a new complexity bound is obtained as
well
The order of the metal to superconductor transition
We present results from large-scale Monte Carlo simulations on the full
Ginzburg-Landau (GL) model, including fluctuations in the amplitude and the
phase of the matter-field, as well as fluctuations of the non-compact
gauge-field of the theory. {}From this we obtain a precise critical value of
the GL parameter \kct separating a first order metal to superconductor
transition from a second order one, \kct = (0.76\pm 0.04)/\sqrt{2}. This
agrees surprisingly well with earlier analytical results based on a disorder
theory of the superconductor to metal transition, where the value
\kct=0.798/\sqrt{2} was obtained. To achieve this, we have done careful
infinite volume and continuum limit extrapolations. In addition we offer a
novel interpretation of \kct, namely that it is also the value separating
\typeI and \typeII behaviour.<Comment: Minor corrections, present version accepted for publication in PR
Superconducting Coherence and the Helicity Modulus in Vortex Line Models
We show how commonly used models for vortex lines in three dimensional
superconductors can be modified to include k=0 excitations. We construct a
formula for the k=0 helicity modulus in terms of fluctuations in the projected
area of vortex loops. This gives a convenient criterion for the presence of
superconducting coherence. We also present Monte Carlo simulations of a
continuum vortex line model for the melting of the Abrikosov vortex lattice in
pure YBCO.Comment: 4 pages RevTeX, 2 eps figures included using eps
Large N study of extreme type II superconductors in a magnetic field
The large N analysis of an extreme type II superconductor is revisited. It is
found that the phase transition is of second-order in dimensions 4 < d < 6. For
the physical dimension d=3 no sign of phase transition is found, contrary to
early claims.Comment: Revtex, 7 pages, no figure
Universal properties for linelike melting of the vortex lattice
Using numerical results obtained within two models describing vortex matter
(interacting elastic lines (Bose model) and uniformly frustrated XY-model) we
establish universal properties of the melting transition within the linelike
regime. These properties, which are captured correctly by both models, include
the scaling of the melting temperature with anisotropy and magnetic field, the
effective line tension of vortices in the liquid regime, the latent heat, the
entropy jump per entanglement length, and relative jump of Josephson energy at
the transition as compared to the latent heat. The universal properties can
serve as experimental fingerprints of the linelike regime of melting.
Comparison of the models allows us to establish boundaries of the linelike
regime in temperature and magnetic field.Comment: Revtex, 12 pages, 2 EPS figure
Critical properties of loop percolation models with optimization constraints
We study loop percolation models in two and in three space dimensions, in
which configurations of occupied bonds are forced to form closed loop. We show
that the uncorrelated occupation of elementary plaquettes of the square and the
simple cubic lattice by elementary loops leads to a percolation transition that
is in the same universality class as the conventional bond percolation. In
contrast to this an optimization constraint for the loop configurations, which
then have to minimize a particular generic energy function, leads to a
percolation transition that constitutes a new universality class, for which we
report the critical exponents. Implication for the physics of solid-on-solid
and vortex glass models are discussed.Comment: 8 pages, 8 figure
Rough Set Approach to Sunspot Classification Problem
Abstract. This paper presents an application of hierarchical learning method based rough set theory to the problem of sunspot classification from satellite images. The Modified Zurich classification scheme [3] is defined by a set of rules containing many complicated and unprecise concepts, which cannot be determined directly from solar images. The idea is to represent the domain knowledge by an ontology of concepts – a treelike structure that describes the relationship between the target concepts, intermediate concepts and attributes. We show that such on-tology can be constructed by a decision tree algorithm and demonstrate the proposed method on the data set containing sunspot extracted from satellite images of solar disk
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