249 research outputs found
Density-functional theory of freezing of vortex-liquid in quasi two-dimensional superconductors
We present a theory of vortex liquid-to-solid transition in homogeneous quasi
2D superconductors. The free energy is written as a functional l of density of
zeroes of the fluctuating order parameter. The transition is weakly first-order
and well below the Hc2(T) line. Transition temperature, discontinuities of the
average Abrikosov ratio and of the average superfluid density, the Debay-Waller
factor and the latent heat are in good agreement with Monte Carlo simulations.
The density is only weakly modulated in the "vortex-solid" phase, consistent
with the density-wave behavior.Comment: 12 pages and 1 figure available upon request, LaTex Version 2.09,
submitted to Phys. Rev. Let
Energy cost associated with vortex crossing in superconductors
Starting from the Ginzburg-Landau free energy of a type II superconductor in
a magnetic field we estimate the energy associated with two vortices crossing.
The calculations are performed by assuming that we are in a part of the phase
diagram where the lowest Landau level approximation is valid. We consider only
two vortices but with two markedly different sets of boundary conditions: on a
sphere and on a plane with quasi-periodic boundary conditions. We find that the
answers are very similar suggesting that the energy is localised to the
crossing point. The crossing energy is found to be field and temperature
dependent -- with a value at the experimentally measured melting line of
, where is the Lindemann
melting criterion parameter. The crossing energy is then used with an extension
of the Marchetti, Nelson and Cates hydrodynamic theory to suggest an
explanation of the recent transport experiments of Safar {{\em et al.}\ }.Comment: 15 pages, RevTex v3.0, followed by 5 postscript figure
First-Order Vortex Lattice Melting and Magnetization of YBaCuO$_{7-\delta}
We present the first non-mean-field calculation of the magnetization
of YBaCuO both above and below the flux-lattice melting
temperature . The results are in good agreement with experiment as a
function of transverse applied field . The effects of fluctuations in both
order parameter and magnetic induction are included in the
Ginzburg-Landau free energy functional: fluctuates within the
lowest Landau level in each layer, while fluctuates uniformly according to
the appropriate Boltzmann factor. The second derivative is predicted to be negative throughout the vortex liquid state and
positive in the solid state. The discontinuities in entropy and magnetization
at melting are calculated to be per flux line per layer and
~emu~cm at a field of 50 kOe.Comment: 11 pages, 4 PostScript figures in one uuencoded fil
Superconducting zero temperature phase transition in two dimensions and in the magnetic field
We derive the Ginzburg-Landau-Wilson theory for the superconducting phase
transition in two dimensions and in the magnetic field. Without disorder the
theory describes a fluctuation induced first-order quantum phase transition
into the Abrikosov lattice. We propose a phenomenological criterion for
determining the transition field and discuss the qualitative effects of
disorder. Comparison with recent experiments on MoGe films is discussed.Comment: 7 pages, 2 figure
Flux Lattice Melting and Lowest Landau Level Fluctuations
We discuss the influence of lowest Landau level (LLL) fluctuations near
H_{c2}(T) on flux lattice melting in YBaCuO (YBCO). We
show that the specific heat step of the flux lattice melting transition in YBCO
single crystals can be attributed largely to the degrees of freedom associated
with LLL fluctuations. These degrees of freedom have already been shown to
account for most of the latent heat. We also show that these results are a
consequence of the correspondence between flux lattice melting and the onset of
LLL fluctuations.Comment: 4 pages, 2 embedded figure
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
Topological Defects in the Abrikosov Lattice of Vortices in Type-II Superconductors
The free energy costs for various defects within an Abrikosov lattice of
vortices are calculated using the lowest Landau level approximation (LLL).
Defect solutions with boundary conditions for lines to meet at a point
(crossing defect) and for lines to twist around each other (braid defect) are
sought for 2, 3, 6, and 12 lines. Many results have been unexpected, including
the nonexistence of a stable two- or three-line braid. This, and the high
energy cost found for a six-line braid lead us to propose that the equilibrium
vortex state is not entangled below the irreversibility line of the high-
superconductors or in a large part of the vortex-liquid phase above this line.
Also, the solution for an infinite straight screw dislocation is found, and
used to give a limiting form for the free energy cost of very large braids.
This depends on the area enclosed by the braid as well as its perimeter length.Comment: 30 pages, 17 Encapsulated PostScript figures, uses Revtex (with epsf
Instabilities and disorder-driven first-order transition of the vortex lattice
Transport studies in a Corbino disk geometry suggest that the Bragg glass
phase undergoes a first-order transition into a disordered solid. This
transition shows a sharp reentrant behavior at low fields. In contrast, in the
conventional strip configuration, the phase transition is obscured by the
injection of the disordered vortices through the sample edges, which results in
the commonly observed vortex instabilities and smearing of the peak effect in
NbSe2 crystals. These features are found to be absent in the Corbino geometry,
in which the circulating vortices do not cross the sample edges.Comment: 12 pages 3 figures. Accepted for publication in Physical Review
Letter
Phase Transitions in a Model Anisotropic High Tc Superconductor
We carry out simulations of the anisotropic uniformly frustrated 3D XY model,
as a model for vortex line fluctuations in high Tc superconductors. We compute
the phase diagram as a function of temperature and anisotropy, for a fixed
applied magnetic field. We find that superconducting coherence parallel to the
field persists into the vortex line liquid state, and that this transition lies
well below the "mean-field" cross-over from the vortex line liquid to the
normal state.Comment: 23 pages + 19 ps figure
Is there a Phase Transition to the Flux Lattice State?
The sharp drops in the resistance and magnetization which are usually
attributed to a phase transition from the vortex liquid state to a crystal
state are explained instead as a crossover between three and two dimensional
behavior, which occurs when the phase coherence length in the liquid becomes
comparable to the sample thickness. Estimates of the width of the crossover
region and the phase coherence length scales are in agreement with experiment.Comment: 4 pages, RevTe
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