124 research outputs found
Boundary effects on the scaling of the superfluid density
We study numerically the influence of the substrate (boundary conditions) on
the finite--size scaling properties of the superfluid density in
superfluid films of thickness within the XY model employing the Monte Carlo
method. Our results suggest that the jump at the
Kosterlitz--Thouless transition temperature depends on the boundary
conditions.Comment: 2 pages, 1 Latex file, 1 postscript figure, 2 style file
Vortex lattce melting in 2D superconductors and Josephson arrays
Monte Carlo simulations of 2D vortex lattice melting in a thin
superconducting film (or alternatively an array of Josephson junctions) are
performed in the London limit. Finite size scaling analyses are used to make a
detailed test of the dislocation mediated melting theory of KTNHY. We find that
the melting transition is weakly first order, with a jump in the shear modulus
very close to that predicted by the KTNHY theory. No hexatic liquid phase is
found.Comment: 12 pages, 4 figures (available on request from
[email protected]), REVTEX [we revise our conclusion on the order of the
melting transition from second to first order - new figure 4 added
Scaling of the superfluid density in superfluid films
We study scaling of the superfluid density with respect to the film thickness
by simulating the model on films of size ()
using the cluster Monte Carlo. While periodic boundary conditions where used in
the planar () directions, Dirichlet boundary conditions where used along the
film thickness. We find that our results can be scaled on a universal curve by
introducing an effective thickness. In the limit of large our scaling
relations reduce to the conventional scaling forms. Using the same idea we find
scaling in the experimental results using the same value of .Comment: 4 pages, one postscript file replaced by one Latex file and 5
postscript figure
Vortex Lattice Melting in 2D Superconducting Networks and Films
We carry out MC studies of 2D superconducting networks, in an applied
magnetic field, for square and honeycomb geometries. We consider both dilute
systems (f=1/q) and systems near full frustration (f=1/2-1/q). For the dilute
case (which models a film as q->infinity), we find two transitions: at
T_c(f)~1/q there is a depinning transition from a pinned to a floating vortex
lattice; at T_m(f)~constant the floating vortex lattice melts into an isotropic
liquid. We analyze this melting according to the Kosterlitz- Thouless theory of
dislocation mediated melting, and find that the melting is weakly first order.
For the case near full frustration, the system can be described in terms of the
density of defects in an otherwise fully frustrated vortex pattern. We find
pinned solid, floating solid, and liquid defect phases, as well as a higher
sharp transition corresponding to the disordering of the fully frustrated
background.Comment: 55 pages, RevTex3.0, 25 figures (available by mail by contacting
[email protected]
Driven Diffusion in the Two-Dimensional Lattice Coulomb Gas; A Model for Flux Flow in Superconducting Networks
We carry out driven diffusion Monte Carlo simulations of the two dimensional
classical lattice Coulomb gas in an applied uniform electric field, as a model
for vortex motion due to an applied d.c. current, in a periodic superconducting
network. A finite-size version of dynamic scaling is used to extract the
dynamic critical exponent z, and infer the non-linear response at the
transition temperature. We consider the f=0 and f=1/2 cases, corresponding to
no applied magnetic field, and to one half flux quantum per unit cell of the
network respectively.Comment: 25 pages, 7 figures (available from [email protected]),
RevTex3.0, URST12
Non-equilibrium transitions in fully frustrated Josephson junction arrays
We study the effect of thermal fluctuations in a fully frustrated Josephson
junction array driven by a current I larger than the apparent critical current
I_c(T). We calculate numerically the behavior of the chiral order parameter of
Z_2 symmetry and the transverse helicity modulus (related to the U(1) symmetry)
as a function of temperature. We find that the Z_2 transition occurs at a
temperature T_{Z_2}(I) which is lower than the temperature T_{U(1)}(I) for the
U(1) transition. Both transitions could be observed experimentally from
measurements of the longitudinal and transverse voltages.Comment: 4 pages, 4 figure
Vortex Line Fluctuations in Model High Temperature Superconductors
We carry out Monte Carlo simulations of the uniformly frustrated 3d XY model
as a model for vortex line fluctuations in a high Tc superconductor. A density
of vortex lines of f=1/25 is considered. We find two sharp phase transitions.
The low T phase is an ordered vortex line lattice. The high T normal phase is a
vortex line liquid with much entangling, cutting, and loop excitations. An
intermediate phase is found which is characterized as a vortex line liquid of
disentangled lines. In this phase, the system displays superconducting
properties in the direction parallel to the magnetic field, but normal behavior
in planes perpendicular to the magnetic field.Comment: 38 pages, LaTeX 15 figures (upon request to
[email protected]
Vortex structure and resistive transitions in high-Tc superconductors
The nature of the resistive transition for a current applied parallel to the
magnetic field in high-Tc materials is investigated by numerical simulation on
the three dimensional Josephson junction array model. It is shown by using
finite size scaling that for samples with disorder the critical temperature Tp
for the c axis resistivity corresponds to a percolation phase transition of
vortex lines perpendicularly to the applied field. The value of Tp is higher
than the critical temperature for j perpendicular to H, but decreases with the
thickness of the sample and with anisotropy. We predict that critical behavior
around Tp should reflect in experimentally accessible quantities, as the I-V
curves.Comment: 8 pages + 6 figure
Helicity Modulus and Meissner Effect in a Fluctuating Type II Superconductor
The helicity modulus for a fluctuating type II superconductor is computed
within the elastic medium approximation, as a probe of superconducting phase
coherence and the Meissner effect in the mixed state. We argue that at the
vortex line lattice melting transition, there remains superconducting coherence
parallel to the applied magnetic field, provided the vortex line liquid retains
a finite shear modulus at finite wavevector.Comment: 10 pages, 0 figures, RevTex3.0, UR-93-ST0
Low-Magnetic Field Critical Behavior in Strongly Type-II Superconductors
A new description is proposed for the low-field critical behavior of type-II
superconductors. The starting point is the Ginzburg-Landau theory in presence
of an external magnetic field H. A set of fictitious vortex variables and a
singular gauge transformation are used to rewrite a finite H Ginzburg-Landau
functional in terms of a complex scalar field of zero average vorticity. The
continuum limit of the transformed problem takes the form of an H = 0
Ginzburg-Landau functional for a charged field coupled to a fictitious `gauge'
potential which arises from long wavelength fluctuations in the background
liquid of field-induced vorticity. A possibility of a novel phase transition
involving zero vorticity degrees of freedom and formation of a uniform
condensate is suggested. A similarity to the superconducting [Higgs]
electrodynamics and the nematic-smectic-A transition in liquid crystals is
noted. The experimental situation is discussed.Comment: 19 pages RevTeX, one figure available by fax [email requests to
[email protected]], to appear in Physical Review B
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