27 research outputs found
On Mean-Field Theory of Quantum Phase Transition in Granular Superconductors
In previous work on quantum phase transition in granular superconductors,
where mean-field theory was used, an assumption was made that the order
parameter as a function of the mean field is a convex up function. Though this
is not always the case in phase transitions, this assumption must be verified,
what is done in this article
Giant Shapiro steps for two-dimensional Josephson-junction arrays with time-dependent Ginzburg-Landau dynamics
Two-dimensional Josephson junction arrays at zero temperature are
investigated numerically within the resistively shunted junction (RSJ) model
and the time-dependent Ginzburg-Landau (TDGL) model with global conservation of
current implemented through the fluctuating twist boundary condition (FTBC).
Fractional giant Shapiro steps are found for {\em both} the RSJ and TDGL cases.
This implies that the local current conservation, on which the RSJ model is
based, can be relaxed to the TDGL dynamics with only global current
conservation, without changing the sequence of Shapiro steps. However, when the
maximum widths of the steps are compared for the two models some qualitative
differences are found at higher frequencies. The critical current is also
calculated and comparisons with earlier results are made. It is found that the
FTBC is a more adequate boundary condition than the conventional uniform
current injection method because it minimizes the influence of the boundary.Comment: 6 pages including 4 figures in two columns, final versio
Domain Walls Motion and Resistivity in a Fully-Frustrated Josephson Array
It is identified numerically that the resistivity of a fully-frustrated
Josephson-junction array is due to motion of domain walls in vortex lattice
rather than to motion of single vortices
Lattice effects on the current-voltage characteristics of superconducting arrays
The lattice effects on the current-voltage characteristics of two-dimensional
arrays of resistively shunted Josephson junctions are investigated. The lattice
potential energies due to the discrete lattice structure are calculated for
several geometries and directions of current injection. We compare the energy
barrier for vortex-pair unbinding with the lattice pinning potential, which
shows that lattice effects are negligible in the low-current limit as well as
in the high-current limit. At intermediate currents, on the other hand, the
lattice potential becomes comparable to the barrier height and the lattice
effects may be observed in the current-voltage characteristics.Comment: 5 pages including 5 figures in two columns, to appear in Phys. Rev.
Towards the characterization of individual users through Web analytics
We perform an analysis of the way individual users navigate in the Web. We
focus primarily in the temporal patterns of they return to a given page. The
return probability as a function of time as well as the distribution of time
intervals between consecutive visits are measured and found to be independent
of the level of activity of single users. The results indicate a rich variety
of individual behaviors and seem to preclude the possibility of defining a
characteristic frequency for each user in his/her visits to a single site.Comment: 8 pages, 4 figures. To appear in Proceeding of Complex'0
Boundary Effects on Dynamic Behavior of Josephson-Junction Arrays
The boundary effects on the current-voltage characteristics in
two-dimensional arrays of resistively shunted Josephson junctions are examined.
In particular, we consider both the conventional boundary conditions (CBC) and
the fluctuating twist boundary conditions (FTBC), and make comparison of the
obtained results. It is observed that the CBC, which have been widely adopted
in existing simulations, may give a problem in scaling, arising from rather
large boundary effects; the FTBC in general turn out to be effective in
reducing the finite-size effects, yielding results with good scaling behavior.
To resolve the discrepancy between the two boundary conditions, we propose that
the proper scaling in the CBC should be performed with the boundary data
discarded: This is shown to give results which indeed scale well and are the
same as those from the FTBC.Comment: RevTex, Final version to appear in Phys. Rev.
Edge effects in a frustrated Josephson junction array with modulated couplings
A square array of Josephson junctions with modulated strength in a magnetic
field with half a flux quantum per plaquette is studied by analytic arguments
and dynamical simulations. The modulation is such that alternate columns of
junctions are of different strength to the rest. Previous work has shown that
this system undergoes an XY followed by an Ising-like vortex lattice
disordering transition at a lower temperature. We argue that resistance
measurements are a possible probe of the vortex lattice disordering transition
as the linear resistance with
at intermediate temperatures due to dissipation at the array
edges for a particular geometry and vanishes for other geometries. Extensive
dynamical simulations are performed which support the qualitative physical
arguments.Comment: 8 pages with figs, RevTeX, to appear in Phys. Rev.
Scaling determination of the nonlinear I-V characteristics for 2D superconducting networks
It is shown from computer simulations that the current-voltage (-)
characteristics for the two-dimensional XY model with resistively-shunted
Josephson junction dynamics and Monte Carlo dynamics obeys a finite-size
scaling form from which the nonlinear - exponent can be determined to
good precision. This determination supports the conclusion , where
is the dynamic critical exponent. The results are discussed in the light of the
contrary conclusion reached by Tang and Chen [Phys. Rev. B {\bf 67}, 024508
(2003)] and the possibility of a breakdown of scaling suggested by Bormann
[Phys. Rev. Lett. {\bf 78}, 4324 (1997)].Comment: 6 pages, to appear in PR
Ferromagnetic phase transition and Bose-Einstein condensation in spinor Bose gases
Phase transitions in spinor Bose gases with ferromagnetic (FM) couplings are
studied via mean-field theory. We show that an infinitesimal value of the
coupling can induce a FM phase transition at a finite temperature always above
the critical temperature of Bose-Einstein condensation. This contrasts sharply
with the case of Fermi gases, in which the Stoner coupling can not lead
to a FM phase transition unless it is larger than a threshold value . The
FM coupling also increases the critical temperatures of both the ferromagnetic
transition and the Bose-Einstein condensation.Comment: 4 pages, 4 figure
Charging Effects and Quantum Crossover in Granular Superconductors
The effects of the charging energy in the superconducting transition of
granular materials or Josephson junction arrays is investigated using a
pseudospin one model. Within a mean-field renormalization-group approach, we
obtain the phase diagram as a function of temperature and charging energy. In
contrast to early treatments, we find no sign of a reentrant transition in
agreement with more recent studies. A crossover line is identified in the
non-superconducting side of the phase diagram and along which we expect to
observe anomalies in the transport and thermodynamic properties. We also study
a charge ordering phase, which can appear for large nearest neighbor Coulomb
interaction, and show that it leads to first-order transitions at low
temperatures. We argue that, in the presence of charge ordering, a non
monotonic behavior with decreasing temperature is possible with a maximum in
the resistance just before entering the superconducting phase.Comment: 15 pages plus 4 fig. appended, Revtex, INPE/LAS-00