8 research outputs found
Nonlinear Viscous Vortex Motion in Two-Dimensional Josephson-Junction Arrays
When a vortex in a two-dimensional Josephson junction array is driven by a
constant external current it may move as a particle in a viscous medium. Here
we study the nature of this viscous motion. We model the junctions in a square
array as resistively and capacitively shunted Josephson junctions and carry out
numerical calculations of the current-voltage characteristics. We find that the
current-voltage characteristics in the damped regime are well described by a
model with a {\bf nonlinear} viscous force of the form , where is the vortex velocity,
is the velocity dependent viscosity and and are
constants for a fixed value of the Stewart-McCumber parameter. This result is
found to apply also for triangular lattices in the overdamped regime. Further
qualitative understanding of the nature of the nonlinear friction on the vortex
motion is obtained from a graphic analysis of the microscopic vortex dynamics
in the array. The consequences of having this type of nonlinear friction law
are discussed and compared to previous theoretical and experimental studies.Comment: 14 pages RevTex, 9 Postscript figure
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
Orientational pinning and transverse voltage: Simulations and experiments in square Josephson junction arrays
We study the dependence of the transport properties of square Josephson
Junctions arrays with the direction of the applied dc current, both
experimentally and numerically. We present computational simulations of
current-voltage curves at finite temperatures for a single vortex in the array
(), and experimental measurements in
arrays under a low magnetic field corresponding to . We find that
the transverse voltage vanishes only in the directions of maximum symmetry of
the square lattice: the [10] and [01] direction (parallel bias) and the [11]
direction (diagonal bias). For orientations different than the symmetry
directions, we find a finite transverse voltage which depends strongly on the
angle of the current. We find that vortex motion is pinned in the [10]
direction (), meaning that the voltage response is insensitive to small
changes in the orientation of the current near . We call this
phenomenon orientational pinning. This leads to a finite transverse critical
current for a bias at and to a transverse voltage for a bias at
. On the other hand, for diagonal bias in the [11] direction the
behavior is highly unstable against small variations of , leading to a
rapid change from zero transverse voltage to a large transverse voltage within
a few degrees. This last behavior is in good agreement with our measurements in
arrays with a quasi-diagonal current drive.Comment: 9 pages, 9 figure
Vortex reflection at boundaries of Josephson-junction arrays
We study the propagation properties of a single vortex in square
Josephson-junction arrays (JJA) with free boundaries and subject to an applied
dc current. We model the dynamics of the JJA by the resistively and
capacitively shunted junction (RCSJ) equations. For zero Stewart-McCumber
parameter we find that the vortex always escapes from the array when
it gets to the boundary. For and for low currents we find
that the vortex escapes, while for larger currents the vortex is reflected as
an antivortex at one edge and the antivortex as a vortex at the other, leading
to a stationary oscillatory state and to a non-zero time-averaged voltage. The
escape and the reflection of a vortex at the array edges are qualitatively
explained in terms of a coarse-grained model of a vortex interacting
logarithmically with its image. We also discuss the case when the free
boundaries are at degrees with respect to the direction of the vortex
motion. Finally, we discuss the effect of self-induced magnetic fields by
taking into account the full-range inductance matrix of the array, and find
qualitatively equivalent results.Comment: 14 pages RevTex, 9 Postscript figure
Transverse phase-locking in fully frustrated Josephson junction arrays: a new type of fractional giant steps
We study, analytically and numerically, phase locking of driven vortex
lattices in fully-frustrated Josephson junction arrays at zero temperature. We
consider the case when an ac current is applied {\it perpendicular} to a dc
current. We observe phase locking, steps in the current-voltage
characteristics, with a dependence on external ac-drive amplitude and frequency
qualitatively different from the Shapiro steps, observed when the ac and dc
currents are applied in parallel. Further, the critical current increases with
increasing transverse ac-drive amplitude, while it decreases for longitudinal
ac-drive. The critical current and the phase-locked current step width,
increase quadratically with (small) amplitudes of the ac-drive. For larger
amplitudes of the transverse ac-signal, we find windows where the critical
current is hysteretic, and windows where phase locking is suppressed due to
dynamical instabilities. We characterize the dynamical states around the
phase-locking interference condition in the curve with voltage noise,
Lyapunov exponents and Poincar\'e sections. We find that zero temperature
phase-locking behavior in large fully frustrated arrays is well described by an
effective four plaquette model.Comment: 12 pages, 11 figure