793 research outputs found
Commensurate and Incommensurate Vortex States in Superconductors with Periodic Pinning Arrays
As a function of applied field, we find a rich variety of ordered and
partially-ordered vortex lattice configurations in systems with square or
triangular arrays of pinning sites. We present formulas that predict the
matching fields at which commensurate vortex configurations occur and the
vortex lattice orientation with respect to the pinning lattice. Our results are
in excellent agreement with recent imaging experiments on square pinning arrays
[K. Harada et al., Science 274, 1167 (1996)].Comment: 9 pages, 3 figures. Accepted to Physical Review
London equation studies of thin-film superconductors with a triangular antidot lattice
We report on a study of vortex pinning in nanoscale antidot defect arrays in
the context of the London Theory. Using a wire network model, we discretize the
array with a fine mesh, thereby providing a detailed treatment of pinning
phenomena. The use of a fine grid has enabled us to examine both circular and
elongated defects, patterned in the form of a rhombus. The latter display
pinning characteristics superior to circular defects constructed with the
similar area. We calculate pinning potentials for defects containing zero and
single quanta, and we obtain a pinning phase diagram for the second matching
field, .Comment: 10 pages and 14 figure
Vortex phase diagram for mesoscopic superconducting disks
Solving numerically the 3D non linear Ginzburg-Landau (GL) equations, we
study equilibrium and nonequilibrium phase transitions between different
superconducting states of mesoscopic disks which are thinner than the coherence
length and the penetration depth. We have found a smooth transition from a
multi-vortex superconducting state to a giant vortex state with increasing both
the disk thickness and the magnetic field. A vortex phase diagram is obtained
which shows, as function of the magnetic field, a re-entrant behavior between
the multi-vortex and the giant vortex state.Comment: 5 figures (post script files) include
Spatio-temporal dynamics and plastic flow of vortices in superconductors with periodic arrays of pinning sites
We present simulations of flux-gradient-driven superconducting rigid vortices
interacting with square and triangular arrays of columnar pinning sites in an
increasing external magnetic field. These simulations allow us to
quantitatively relate spatio-temporal microscopic information of the vortex
lattice with typically measured macroscopic quantities, such as the
magnetization . The flux lattice does not become completely commensurate
with the pinning sites throughout the sample at the magnetization matching
peaks, but forms a commensurate lattice in a region close to the edge of the
sample. Matching fields related to unstable vortex configurations do not
produce peaks in . We observe a variety of evolving complex flux
profiles, including flat terraces or plateaus separated by winding
current-carrying strings and, near the peaks in , plateaus only in
certain regions, which move through the sample as the field increases
Shapiro steps in a superconducting film with an antidot lattice
Shapiro voltage steps at voltages V_n=nV_0 (n integer) have been observed in
the voltage-current characteristics of a superconducting film with a square
lattice of perforating microholes (antidots)in the presence of radiofrequent
radiation. These equidistant steps appear at the second matching field H_2 when
the flow of the interstitial vortex lattice in the periodic potential created
by the antidots and the vortices trapped by them, is in phase with the applied
rf frequency. Therefore, the observation of Shapiro steps clearly reveals the
presence of mobile intersitial vortices in superconducting films with regular
pinning arrays. The interstitial vortices, moved by the driving current,
coexist with immobile vortices strongly pinned at the antidots.Comment: 6 pages text, 3 EPS figures, RevTeX, accepted for publication in PRB
Rapid Communication
Order in driven vortex lattices in superconducting Nb films with nanostructured pinning potentials
Driven vortex lattices have been studied in a material with strong pinning,
such as Nb films. Samples in which natural random pinning coexists with
artificial ordered arrays of defects (submicrometric Ni dots) have been
fabricated with different geometries (square, triangular and rectangular).
Three different dynamic regimes are found: for low vortex velocities, there is
a plastic regime in which random defects frustrate the effect of the ordered
array; then, for vortex velocities in the range 1-100 m/s, there is a sudden
increase in the interaction between the vortex lattice and the ordered dot
array, independent on the geometry. This effect is associated to the onset of
quasi long range order in the vortex lattice leading to an increase in the
overlap between the vortex lattice and the magnetic dots array. Finally, at
larger velocities the ordered array-vortex lattice interaction is suppresed
again, in agreement with the behavior found in numerical simulations.Comment: 8 text pages + 4 figure
Vortex Structure Around a Magnetic Dot in Planar Superconductors
The problem of the giant vortex state around a magnetic dot which is embedded
in a superconducting film is investigated. The full non-linear, self-consistent
Ginzburg-Landau equations are solved numerically in order to calculate the free
energy, the order parameter of the host superconductor, the internal magnetic
field due to the supercurrents, the corresponding current density, the
magnetization probed in the vicinity of the dot, and the normal electron
density as a function of the various parameters of the system. We find that, as
we increase the magnetic moment of the dot, higher flux quanta vortex states
become energetically more favorable, as they can better compete with the
external magnetic field via the Meissner effect. In addition to that, they
progressively become closer to each other in energy with direct experimental
consequences, i.e. physical quantities like magnetization may fluctuate when
measured, for example, as a function of a uniform external magnetic field.Comment: text 21 pages (REVTEX), 8 figures available upon reques
Louis-Ferdinand CĂ©line, literary genius or national pariah? Defining moral parameters for influential cultural figures, post- Charlie Hebdo
In January 2011 the French Minister of Culture, Frédéric Mitterrand, withdrew Louis-Ferdinand Céline from a list of famous French authors specifically selected for a national celebration of culture. This bold decision polarized opinion: while many welcomed Mitterrand’s intervention, a number of prominent writers, some of them Jewish, opposed it on the grounds that Céline’s abhorrent political beliefs – expressed in three anti-Semitic pamphlets and his flirtation with Nazism- should in no way detract from his literary genius. In the light of this controversy, and of the rise in anti-Semitism following the Charlie Hebdo attacks of January 2015, this paper proposes Céline as a vital case study of the moral parameters a democratic nation should apply to a culturally important figure whose political views are deemed unacceptably reactionary
Incommensuration Effects and Dynamics in Vortex Chains
We examine the motion of one-dimensional (1D) vortex matter embedded in a 2D
vortex system with weak pinning using numerical simulations. We confirm the
conjecture of Matsuda et al. [Science 294, 2136 (2001)] that the onset of the
temperature induced motion of the chain is due to an incommensuration effect of
the chain with the periodic potential created by the bulk vortices. In
addition, under an applied driving force we find a two stage depinning
transition, where the initial depinning of the vortex chain occurs through
soliton like pulses. When an ac drive is added to the dc drive, we observe
phase locking of the moving vortex chain.Comment: 4 pages, 4 postscript figure
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