306 research outputs found
Josephson junction between anisotropic superconductors
The sin-Gordon equation for Josephson junctions with arbitrary misaligned
anisotropic banks is derived. As an application, the problem of Josephson
vortices at twin planes of a YBCO-like material is considered. It is shown that
for an arbitrary orientation of these vortices relative to the crystal axes of
the banks, the junctions should experience a mechanical torque which is
evaluated. This torque and its angular dependence may, in principle, be
measured in small fields, since the flux penetration into twinned crystals
begins with nucleation of Josephson vortices at twin planes.Comment: 6 page
Electromagnetic waves in a Josephson junction in a thin film
We consider a one-dimensional Josephson junction in a superconducting film
with the thickness that is much less than the London penetration depth. We
treat an electromagnetic wave propagating along this tunnel contact. We show
that the electrodynamics of a Josephson junction in a thin film is nonlocal if
the wave length is less than the Pearl penetration depth. We find the
integro-differential equation determining the phase difference between the two
superconductors forming the tunnel contact. We use this equation to calculate
the dispersion relation for an electromagnetic wave propagating along the
Josephson junction. We find that the frequency of this wave is proportional to
the square root of the wave vector if the wave length is less than the Pearl
penetration depth.Comment: 12 pages, a figure is included as a uuencodeded postscript file,
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Shapiro steps in Josephson junctions with alternating critical current density
We treat theoretically Shapiro steps in tunnel Josephson junctions with
spatially alternating critical current density. Explicit analytical formulas
for the width of the first integer (normal) and half-integer (anomalous)
Shapiro steps are derived for short junctions. We develop coarse-graining
approach, which describes Shapiro steps in the voltage-current curves of the
asymmetric grain boundaries in YBCO thin films and different
superconductor-ferromagnet-superconductor Josephson-type heterostructures.Comment: 5 pages, 3 figures, accepted for publication in Phys. Rev.
High-field vortices in Josephson junctions with alternating critical current density
We study long Josephson junctions with the critical current density
alternating along the junction. New equilibrium states, which we call the field
synchronized or FS states, are shown to exist if the applied field is from
narrow intervals centered around equidistant series of resonant fields, .
The values of are much higher than the flux penetration field, . The
flux per period of the alternating critical current density, , is fixed
for each of the FS states. In the -th FS state the value of is
equal to an integer amount of flux quanta, . Two types of
single Josephson vortices carrying fluxes or/and can exist
in the FS states. Specific stepwise resonances in the current-voltage
characteristics are caused by periodic motion of these vortices between the
edges of the junction.Comment: 4 pages, 5 figure
Flux creep in type-II superconductors: self-organized criticality approach
We consider the current density distribution function of a flux creep regime
in type-II superconductors by mapping the flux creep process to the dynamics of
a model with a self-organized criticality. We use an extremal Robin Hood type
model which evolves to Been's type critical state to treat magnetic flux
penetration into a superconductor and derive an analog of the current-voltage
characteristics in the flux creep region.Comment: 7 pages, 7 figure
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.
Role of Interfaces in the Proximity Effect in Anisotropic Superconductors
We report measurements of the critical temperature of YBCO-Co doped YBCO
Superconductor-Normal bilayer films. Depending on the morphology of the S-N
interface, the coupling between S and N layers can be turned on to depress the
critical temperature of S by tens of degrees, or turned down so the layers
appear almost totally decoupled. This novel effect can be explained by the
mechanism of quasiparticle transmission into an anisotropic superconductor.Comment: 13 pages, 3 figure
Applicability of layered sine-Gordon models to layered superconductors: II. The case of magnetic coupling
In this paper, we propose a quantum field theoretical renormalization group
approach to the vortex dynamics of magnetically coupled layered
superconductors, to supplement our earlier investigations on the
Josephson-coupled case. We construct a two-dimensional multi-layer sine-Gordon
type model which we map onto a gas of topological excitations. With a special
choice of the mass matrix for our field theoretical model, vortex dominated
properties of magnetically coupled layered superconductors can be described.
The well known interaction potentials of fractional flux vortices are
consistently obtained from our field-theoretical analysis, and the physical
parameters (vortex fugacity and temperature parameter) are also identified. We
analyse the phase structure of the multi-layer sine--Gordon model by a
differential renormalization group method for the magnetically coupled case
from first principles. The dependence of the transition temperature on the
number of layers is found to be in agreement with known results based on other
methods.Comment: 7 pages, 1 figure, published in J. Phys.: Condens. Matte
Magnetic Quantum Dot: A Magnetic Transmission Barrier and Resonator
We study the ballistic edge-channel transport in quantum wires with a
magnetic quantum dot, which is formed by two different magnetic fields B^* and
B_0 inside and outside the dot, respectively. We find that the electron states
located near the dot and the scattering of edge channels by the dot strongly
depend on whether B^* is parallel or antiparallel to B_0. For parallel fields,
two-terminal conductance as a function of channel energy is quantized except
for resonances, while, for antiparallel fields, it is not quantized and all
channels can be completely reflected in some energy ranges. All these features
are attributed to the characteristic magnetic confinements caused by nonuniform
fields.Comment: 4 pages, 4 figures, to be published in Physical Review Letter
Anomalous Periodicity of the Current-Phase Relationship of Grain-Boundary Josephson Junctions in High-Tc Superconductors
The current-phase relation (CPR) for asymmetric 45 degree Josephson junctions
between two d-wave superconductors has been predicted to exhibit an anomalous
periodicity. We have used the single-junction interferometer to investigate the
CPR for this kind of junctions in YBCO thin films. Half-fluxon periodicity has
been experimentally found, providing a novel source of evidence for the d-wave
symmetry of the pairing state of the cuprates.Comment: 4 pages, 5 figure
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