48 research outputs found
Reverse-domain superconductivity in superconductor-ferromagnet hybrids: effect of a vortex-free channel on the symmetry of I-V characteristics
We demonstrate experimentally that the presence of a single domain wall in an
underlying ferromagnetic BaFe_{12}O_{19} substrate can induce a considerable
asymmetry in the current (I) - voltage (V) characteristics of a superconducting
Al bridge. The observed diode-like effect, i.e. polarity-dependent critical
current, is associated with the formation of a vortex-free channel inside the
superconducting area which increases the total current flowing through the
superconducting bridge without dissipation. The vortex-free region appears only
for a certain sign of the injected current and for a limited range of the
external magnetic field
Superconducting thin rings with finite penetration depth
Recently Babaei Brojeny and Clem [Phys. Rev. B 68, 174514 (2003)] considered
superconducting thin-film rings in perpendicular magnetic fields in the ideal
Meissner state with negligibly small magnetic penetration depth and presented
useful analytical limiting expressions and numerical results for the
magnetic-field and sheet-current profiles, trapped magnetic flux,
self-inductance, magnetic moment, and focusing of magnetic flux into the hole
when no net current flows in the ring. The present paper generalizes all these
results to rings with arbitrary values of the two-dimensional effective
penetration depth \Lambda = \lambda^2 /d (\lambda is the London depth and d <
\lambda/2 the film thickness) using a straightforward matrix inversion method.
We also present results for the energy of a superconducting ring as a function
of the applied magnetic induction B_a and the quantum number N defining the
size of the fluxoid N \phi_0 trapped in the hole.Comment: with 19 figures, gives 11.5 page
Nonlocal vs local vortex dynamics in the transversal flux transformer effect
In this follow-up to our recent Letter [F. Otto et al., Phys. Rev. Lett. 104,
027005 (2010)], we present a more detailed account of the superconducting
transversal flux transformer effect (TFTE) in amorphous (a-)NbGe nanostructures
in the regime of strong nonequilibrium in local vortex motion. Emphasis is put
on the relation between the TFTE and local vortex dynamics, as the former turns
out to be a reliable tool for determining the microscopic mechanisms behind the
latter. By this method, a progression from electron heating at low temperatures
T to the Larkin-Ovchinnikov effect close to the transition temperature Tc is
traced over a range 0.26 < T/Tc < 0.95. This is represented by a number of
relevant parameters such as the vortex transport entropy related to the
Nernst-like effect at low T, and a nonequilibrium magnetization enhancement
close to Tc. At intermediate T, the Larkin-Ovchinnikov effect is at high
currents modified by electron heating, which is clearly observed only in the
TFTE
Geometry-induced reduction of the critical current in superconducting nanowires
Reduction of the critical current in narrow superconducting NbN lines with
sharp and rounded bends with respect to the critical current in straight lines
was studied at different temperatures. We compare our experimental results with
the reduction expected in the framework of the London model and the
Ginsburg-Landau model. We have experimentally found that the reduction is
significantly less than either model predicts. We also show that in our NbN
lines the bends mostly contribute to the reduction of the critical current at
temperatures well below the superconducting transition temperature
Suppression of surface barrier in superconductors by columnar defects
We investigate the influence of columnar defects in layered superconductors
on the thermally activated penetration of pancake vortices through the surface
barrier. Columnar defects, located near the surface, facilitate penetration of
vortices through the surface barrier, by creating ``weak spots'', through which
pancakes can penetrate into the superconductor. Penetration of a pancake
mediated by an isolated column, located near the surface, is a two-stage
process involving hopping from the surface to the column and the detachment
from the column into the bulk; each stage is controlled by its own activation
barrier. The resulting effective energy is equal to the maximum of those two
barriers. For a given external field there exists an optimum location of the
column for which the barriers for the both processes are equal and the
reduction of the effective penetration barrier is maximal. At high fields the
effective penetration field is approximately two times smaller than in
unirradiated samples. We also estimate the suppression of the effective
penetration field by column clusters. This mechanism provides further reduction
of the penetration field at low temperatures.Comment: 8 pages, 9 figures, submitted to Phys. Rev.
Considerable enhancement of the critical current in a superconducting film by magnetized magnetic strip
We show that a magnetic strip on top of a superconducting strip magnetized in
a specified direction may considerably enhance the critical current in the
sample. At fixed magnetization of the magnet we observed diode effect - the
value of the critical current depends on the direction of the transport
current. We explain these effects by a influence of the nonuniform magnetic
field induced by the magnet on the current distribution in the superconducting
strip. The experiment on a hybrid Nb/Co structure confirmed the predicted
variation of the critical current with a changing value of magnetization and
direction of the transport current.Comment: 6 pages, 7 figure
Rearrangement of the vortex lattice due to instabilities of vortex flow
With increasing applied current we show that the moving vortex lattice
changes its structure from a triangular one to a set of parallel vortex rows in
a pinning free superconductor. This effect originates from the change of the
shape of the vortex core due to non-equilibrium effects (similar to the
mechanism of vortex motion instability in the Larkin-Ovchinnikov theory). The
moving vortex creates a deficit of quasiparticles in front of its motion and an
excess of quasiparticles behind the core of the moving vortex. This results in
the appearance of a wake (region with suppressed order parameter) behind the
vortex which attracts other vortices resulting in an effective
direction-dependent interaction between vortices. When the vortex velocity
reaches the critical value quasi-phase slip lines (lines with fast vortex
motion) appear which may coexist with slowly moving vortices between such
lines. Our results are found within the framework of the time-dependent
Ginzburg-Landau equations and are strictly valid when the coherence length
is larger or comparable with the decay length of the
non-equilibrium quasiparticle distribution function. We qualitatively explain
experiments on the instability of vortex flow at low magnetic fields when the
distance between vortices . We speculate that a
similar instability of the vortex lattice should exist for even when
.Comment: 10 pages, 11 figure
Heating of quasiparticles driven by oscillations of the order parameter in short superconducting microbridges
We predict 'heating' of quasiparticles driven by order parameter oscillations
in the resistive state of short superconducting microbridges. The finite
relaxation time of the magnitude of the order parameter and the
dependence of the spectral functions both on and the supervelocity
are the origin of this effect. Our result is opposite to those of Aslamazov
and Larkin (Zh. Eks. Teor. Fiz. {\bf 70}, 1340 (1976)) and Schmid, Schon and
Tinkham (Phys. Rev. B {\bf 21} 5076 (1980)) where 'cooling' of quasiparticles
was found.Comment: 7 pages, 6 figure