88 research outputs found
Superconducting films with antidot arrays - novel behavior of the critical current
Novel behavior of the critical current density of a regularly
perforated superconducting film is found, as a function of applied magnetic
field . Previously pronounced peaks of at matching fields were
always found to decrease with increasing . Here we found a {\it reversal of
this behavior} for particular geometrical parameters of the antidot lattice
and/or temperature. This new phenomenon is due to a strong ``caging'' of
interstitial vortices between the pinned ones. We show that this vortex-vortex
interaction can be further tailored by an appropriate choice of the
superconducting material, described by the Ginzburg-Landau parameter .
In effective type-I samples we predict that the peaks in at the
matching fields are transformed into a {\it step-like behavior}.Comment: 5 pages, 4 figure
Fluxonic Cellular Automata
We formulate a new concept for computing with quantum cellular automata
composed of arrays of nanostructured superconducting devices. The logic states
are defined by the position of two trapped flux quanta (vortices) in a 2x2
blind-hole-matrix etched on a mesoscopic superconducting square. Such small
computational unit-cells are well within reach of current fabrication
technology. In an array of unit-cells, the vortex configuration of one cell
influences the penetrating flux lines in the neighboring cell through the
screening currents. Alternatively, in conjoined cells, the information transfer
can be strengthened by the interactions between the supercurrents in adjacent
cells. Here we present the functioning logic gates based on this fluxonic
cellular automata (FCA), where the logic operations are verified through
theoretical simulations performed in the framework of the time-dependent
Ginzburg-Landau theory. The input signals are defined by current loops placed
on top of the two diagonal blind holes of the input cell. For given
current-polarization, external flux lines are attracted or repelled by the
loops, forming the '0' or '1' configuration. The read-out technology may be
chosen from a large variety of modern vortex imaging methods, transport and
LDOS measurements.Comment: Featured on the cover page of APL, November 2007 issu
Vortex states in nanoscale superconducting squares: the influence of quantum confinement
Bogoliubov-de Gennes theory is used to investigate the effect of the size of
a superconducting square on the vortex states in the quantum confinement
regime. When the superconducting coherence length is comparable to the Fermi
wavelength, the shape resonances of the superconducting order parameter have
strong influence on the vortex configuration. Several unconventional vortex
states, including asymmetric ones, giant multi-vortex combinations, and states
comprising giant antivortex, were found as ground states and their stability
was found to be very sensitive on the value of , the size of the
sample , and the magnetic flux . By increasing the temperature and/or
enlarging the size of the sample, quantum confinement is suppressed and the
conventional mesoscopic vortex states as predicted by the Ginzburg-Laudau (GL)
theory are recovered. However, contrary to the GL results we found that the
states containing symmetry-induced vortex-antivortex pairs are stable over the
whole temperature range. It turns out that the inhomogeneous order parameter
induced by quantum confinement favors vortex-antivortex molecules, as well as
giant vortices with a rich structure in the vortex core - unattainable in the
GL domain
Realization of Artificial Ice Systems for Magnetic Vortices in a Superconducting MoGe Thin-film with Patterned Nanostructures
We report an anomalous matching effect in MoGe thin films containing pairs of
circular holes arranged in such a way that four of those pairs meet at each
vertex point of a square lattice. A remarkably pronounced fractional matching
was observed in the magnetic field dependences of both the resistance and the
critical current. At the half matching field the critical current can be even
higher than that at zero field. This has never been observed before for
vortices in superconductors with pinning arrays. Numerical simulations within
the nonlinear Ginzburg-Landau theory reveal a square vortex ice configuration
in the ground state at the half matching field and demonstrate similar
characteristic features in the field dependence of the critical current,
confirming the experimental realization of an artificial ice system for
vortices for the first time.Comment: To appear in Phys. Rev. Let
Vortex configurations and critical parameters in superconducting thin films containing antidot arrays: Nonlinear Ginzburg-Landau theory
Using the non-linear Ginzburg-Landau (GL) theory, we obtain the possible
vortex configurations in superconducting thin films containing a square lattice
of antidots. The equilibrium structural phase diagram is constructed which
gives the different ground-state vortex configurations as function of the size
and periodicity of the antidots for a given effective GL parameter
. Giant-vortex states, combination of giant- and multi-vortex
states, as well as symmetry imposed vortex-antivortex states are found to be
the ground state for particular geometrical parameters of the sample. The
antidot occupation number is calculated as a function of related
parameters and comparison with existing expressions for the saturation number
and with experimental results is given. For a small radius of antidots a
triangular vortex lattice is obtained, where some of the vortices are pinned by
the antidots and some of them are located between them. Transition between the
square pinned and triangular vortex lattices is given for different values of
the applied field. The enhanced critical current at integer and rational
matching fields is found, where the level of enhancement at given magnetic
field directly depends on the vortex-occupation number of the antidots. For
certain parameters of the antidot lattice and/or temperature the critical
current is found to be larger for higher magnetic fields.
Superconducting/normal phase boundary exhibits different regimes as
antidots are made larger, and we transit from a plain superconducting film to a
thin-wire superconducting network. Presented results are in good agreement with
available experiments and suggest possible new experiments.Comment: 15 pages and 20 figure
Mesoscopic field and current compensator based on a hybrid superconductor-ferromagnet structure
A rather general enhancement of superconductivity is demonstrated in a hybrid
structure consisting of submicron superconducting (SC) sample combined with an
in-plane ferromagnet (FM). The superconducting state resists much higher
applied magnetic fields for both perpendicular polarities, as applied field is
screened by the FM. In addition, FM induces (in the perpendicular direction to
its moment) two opposite current-flows in the SC plane, under and aside the
magnet, respectively. Due to the compensation effects, superconductivity
persists up to higher applied currents. With increasing current, the sample
undergoes SC-"resistive"-normal state transitions through a mixture of
vortex-antivortex and phase-slip phenomena.Comment: 4 pages, 5 figures, to appear in Phys. Rev. Let
Vortex-vortex interaction in thin superconducting films
The properties of vortices in superconducting thin films are revisited. The
interaction between two Pearl vortices in an infinite film is approximated at
all distances by a simple expression. The interaction of a vortex with a
regular lattice of real or image vortices is given. The two spring constants
are calculated that one vortex in the vortex lattice feels when the surrounding
vortices are rigidly pinned or are free. The modification of these London
results by the finite size of real films is obtained. In finite films, the
interaction force between two vortices is not a central force but depends on
both vortex positions, not only on their distance. At the film edges the
interaction energy is zero and the interaction force is peaked. Even far from
the edges the vortex interaction considerably deviates from the Pearl result
and is always smaller than it.Comment: 9 pages, 6 figure
Predicted field-dependent increase of critical currents in asymmetric superconducting nanocircuits
The critical current of a thin superconducting strip of width much larger
than the Ginzburg-Landau coherence length but much smaller than the Pearl
length is maximized when the strip is straight with
defect-free edges. When a perpendicular magnetic field is applied to a long
straight strip, the critical current initially decreases linearly with but
then decreases more slowly with when vortices or antivortices are forced
into the strip. However, in a superconducting strip containing sharp 90-degree
or 180-degree turns, the zero-field critical current at H=0 is reduced because
vortices or antivortices are preferentially nucleated at the inner corners of
the turns, where current crowding occurs. Using both analytic London-model
calculations and time-dependent Ginzburg-Landau simulations, we predict that in
such asymmetric strips the resulting critical current can be {\it increased} by
applying a perpendicular magnetic field that induces a current-density
contribution opposing the applied current density at the inner corners. This
effect should apply to all turns that bend in the same direction.Comment: Introduction rewritten to include additional references, 17 pages, 14
figure
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