15 research outputs found
Mode locking of vortex matter driven through mesoscopic channels
We investigated the driven dynamics of vortices confined to mesoscopic flow
channels by means of a dc-rf interference technique. The observed mode-locking
steps in the -curves provide detailed information on how the number of rows
and lattice structure in the channel change with magnetic field. Minima in flow
stress occur when an integer number of rows is moving coherently, while maxima
appear when incoherent motion of mixed and row configurations is
predominant. Simulations show that the enhanced pinning at mismatch originates
from quasi-static fault zones with misoriented edge dislocations induced by
disorder in the channel edges.Comment: some minor changes were made, 4 pages, 4 figures, accepted for
publication in Phys. Rev. Let
Directional vortex motion guided by artificially induced mesoscopic potentials
Rectangular pinning arrays of Ni dots define a potential landscape for vortex
motion in Nb films. Magnetotransport experiments in which two in-plane
orthogonal electrical currents are injected simultaneously allow selecting the
direction and magnitude of the Lorentz force on the vortex-lattice, thus
providing the angular dependence of the vortex motion. The background
dissipation depends on angle at low magnetic fields, which is progressively
smeared out with increasing field. The periodic potential locks in the vortex
motion along channeling directions. Because of this, vortex-lattice direction
of motion is up to 85o away from the applied Lorentz force direction.Comment: PDF file includes figure
Anisotropic vortex pinning in superconductors with a square array of rectangular submicron holes
We investigate vortex pinning in thin superconducting films with a square
array of rectangular submicron holes ("antidots"). Two types of antidots are
considered: antidots fully perforating the superconducting film, and "blind
antidots", holes that perforate the film only up to a certain depth. In both
systems, we observe a distinct anisotropy in the pinning properties, reflected
in the critical current Ic, depending on the direction of the applied
electrical current: parallel to the long side of the antidots or perpendicular
to it. Although the mechanism responsible for the effect is very different in
the two systems, they both show a higher critical current and a sharper
IV-transition when the current is applied along the long side of the
rectangular antidots
Temperature dependence and mechanisms for vortex pinning by periodic arrays of Ni dots in Nb films
Pinning interactions between superconducting vortices in Nb and magnetic Ni
dots were studied as a function of current and temperature to clarify the
nature of pinning mechanisms. A strong current dependence is found for a square
array of dots, with a temperature dependent optimum current for the observation
of periodic pinning, that decreases with temperature as (1-T/Tc)3/2. This same
temperature dependence is found for the critical current at the first matching
field with a rectangular array of dots. The analysis of these results allows to
narrow the possible pinning mechanisms to a combination of two: the interaction
between the vortex and the magnetic moment of the dot and the proximity effect.
Moreover, for the rectangular dot array, the temperature dependence of the
crossover between the low field regime with a rectangular vortex lattice to the
high field regime with a square configuration has been studied. It is found
that the crossover field increases with decreasing temperature. This dependence
indicates a change in the balance between elastic and pinning energies,
associated with dynamical effects of the vortex lattice in the high field
range.Comment: 12 text pages (revtex), 6 figures (1st jpeg, 2nd-6th postscript)
accepted in Physical Review
Disorder Effect on the Vortex Pinning by the Cooling Process Control in the Organic Superconductor -(BEDT-TTF)Cu[N(CN)]Br
We investigate the influence of disorders in terminal ethylene groups of
BEDT-TTF molecules (ethylene-disorders) on the vortex pinning of the organic
superconductor -(BEDT-TTF)Cu[N(CN)]Br. Magnetization
measurements are performed under different cooling-processes. The second peak
in the magnetization hysteresis curve is observed for all samples studied, and
the hysteresis width of the magnetization becomes narrower by cooling faster.
In contradiction to the simple pinning effect of disorder, this result shows
the suppression of the vortex pinning force by introducing more
ethylene-disorders. The ethylene-disorder domain model is proposed for
explaining the observed result. In the case of the system containing a moderate
number of the ethylene-disorders, the disordered molecules form a domain
structure and it works as an effective pinning site. On the contrary, an excess
number of the ethylene-disorders may weaken the effect of the domain structure,
which results in the less effective pinning force on the vortices.Comment: 6 pages, 6 figure
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
Commensurate and Incommensurate Vortex Lattice Melting in Periodic Pinning Arrays
We examine the melting of commensurate and incommensurate vortex lattices
interacting with square pinning arrays through the use of numerical
simulations. For weak pinning strength in the commensurate case we observe an
order-order transition from a commensurate square vortex lattice to a
triangular floating solid phase as a function of temperature. This floating
solid phase melts into a liquid at still higher temperature. For strong pinning
there is only a single transition from the square pinned lattice to the liquid
state. For strong pinning in the incommensurate case, we observe a multi-stage
melting in which the interstitial vortices become mobile first, followed by the
melting of the entire lattice, consistent with recent imaging experiments. The
initial motion of vortices in the incommensurate phase occurs by an exchange
process of interstitial vortices with vortices located at the pinning sites. We
have also examined the vortex melting behavior for higher matching fields and
find that a coexistence of a commensurate pinned vortex lattice with an
interstitial vortex liquid occurs while at higher temperatures the entire
vortex lattice melts. For triangular arrays at incommensurate fields higher
than the first matching field we observe that the initial vortex motion can
occur through a novel correlated ring excitation where a number of vortices can
rotate around a pinned vortex. We also discuss the relevance of our results to
recent experiments of colloidal particles interacting with periodic trap
arrays.Comment: 8 figure