67 research outputs found
Lattice effects and current reversal in superconducting ratchets
Competition between the vortex lattice and a lattice of asymmetric artificial
defects is shown to play a crucial role in ratchet experiments in
superconducting films. We present a novel and collective mechanism for current
reversal based on a reconfiguration of the vortex lattice. In contrast to
previous models of vortex current reversal, the mechanism is based on the
global response of the vortex lattice to external forces.Comment: 12 pages, 7 figure
Quantifying nutrient inputs by gulls to a fluctuating lake, aided by movement ecology methods
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
Crossed-ratchet effects and domain wall geometrical pinning
The motion of a domain wall in a two dimensional medium is studied taking
into account the internal elastic degrees of freedom of the wall and
geometrical pinning produced both by holes and sample boundaries. This study is
used to analyze the geometrical conditions needed for optimizing crossed
ratchet effects in periodic rectangular arrays of asymmetric holes, recently
observed experimentally in patterned ferromagnetic films. Geometrical
calculations and numerical simulations have been used to obtain the anisotropic
critical fields for depinning flat and kinked walls in rectangular arrays of
triangles. The aim is to show with a generic elastic model for interfaces how
to build a rectifier able to display crossed ratchet effects or effective
potential landscapes for controlling the motion of interfaces or invasion
fronts.Comment: 13 pages, 18 figure
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
Functional connectivity network between terrestrial and aquatic habitats by a generalist waterbird, and implications for biovectoring
Spatial patterns of weed dispersal by wintering gulls within and beyond an agricultural landscape
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
Flux flow resistivity and vortex viscosity of high-Tc films
The flux flow regime of high-T samples of different normal state
resistivities is studied in the temperature range where the sign of the Hall
effect is reversed. The scaling of the vortex viscosity with normal state
resistivity is consistent with the Bardeen-Stephen theory. Estimates of the
influence of possible mechanisms suggested for the sign reversal of the Hall
effect are also given.Comment: 3 pages. 4 figures upon reques
Topologically protected superconducting ratchet effect generated by spin-ice nanomagnets
We have designed, fabricated and tested a robust superconducting ratchet device based on topologically frustrated spin ice nanomagnets. The device is made of a magnetic Co honeycomb array embedded in a superconducting Nb film. This device is based on three simple mechanisms: (i) the topology of the Co honeycomb array frustrates in-plane magnetic configurations in the array yielding a distribution of magnetic charges which can be ordered or disordered with in-plane magnetic fields, following spin ice rules; (ii) the local vertex magnetization, which consists of a magnetic half vortex with two charged magnetic Neel walls; (iii) the interaction between superconducting vortices and the asymmetric potentials provided by the Neel walls. The combination of these elements leads to a superconducting ratchet effect. Thus, superconducting vortices driven by alternating forces and moving on magnetic half vortices generate a unidirectional net vortex flow. This ratchet effect is independent of the distribution of magnetic charges in the array
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