362 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,
ReVTe
Semantics and Proof Theory of the Epsilon Calculus
The epsilon operator is a term-forming operator which replaces quantifiers in
ordinary predicate logic. The application of this undervalued formalism has
been hampered by the absence of well-behaved proof systems on the one hand, and
accessible presentations of its theory on the other. One significant early
result for the original axiomatic proof system for the epsilon-calculus is the
first epsilon theorem, for which a proof is sketched. The system itself is
discussed, also relative to possible semantic interpretations. The problems
facing the development of proof-theoretically well-behaved systems are
outlined.Comment: arXiv admin note: substantial text overlap with arXiv:1411.362
Interference patterns of multifacet 20x(0-pi-) Josephson junctions with ferromagnetic barrier
We have realized multifacet Josephson junctions with periodically alternating
critical current density (MJJs) using
superconductor-insulator-ferromagnet-superconductor heterostructures. We show
that anomalous features of critical current vs. applied magnetic field,
observed also for other types of MJJs, are caused by a non-uniform flux density
(parallel to the barrier) resulting from screening currents in the electrodes
in the presence of a (parasitic) off-plane field component.Comment: submitted to PR
Visualizing supercurrents in ferromagnetic Josephson junctions with various arrangements of 0 and \pi segments
Josephson junctions with ferromagnetic barrier can have positive or negative
critical current depending on the thickness of the ferromagnetic layer.
Accordingly, the Josephson phase in the ground state is equal to 0 (a
conventional or 0 junction) or to ( junction). When 0 and
segments are joined to form a "0- junction", spontaneous supercurrents
around the 0- boundary can appear. Here we report on the visualization of
supercurrents in superconductor-insulator-ferromagnet-superconductor (SIFS)
junctions by low-temperature scanning electron microscopy (LTSEM). We discuss
data for rectangular 0, , 0-, 0--0 and 20 \times 0-
junctions, disk-shaped junctions where the 0- boundary forms a ring, and
an annular junction with two 0- boundaries. Within each 0 or segment
the critical current density is fairly homogeneous, as indicated both by
measurements of the magnetic field dependence of the critical current and by
LTSEM. The parts have critical current densities up to
35\units{A/cm^2} at T = 4.2\units{K}, which is a record value for SIFS
junctions with a NiCu F-layer so far. We also demonstrate that SIFS technology
is capable to produce Josephson devices with a unique topology of the 0-
boundary.Comment: 29 pages, 8 figure
The Epsilon Calculus and Herbrand Complexity
Hilbert's epsilon-calculus is based on an extension of the language of
predicate logic by a term-forming operator . Two fundamental
results about the epsilon-calculus, the first and second epsilon theorem, play
a role similar to that which the cut-elimination theorem plays in sequent
calculus. In particular, Herbrand's Theorem is a consequence of the epsilon
theorems. The paper investigates the epsilon theorems and the complexity of the
elimination procedure underlying their proof, as well as the length of Herbrand
disjunctions of existential theorems obtained by this elimination procedure.Comment: 23 p
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.
Experimental evidence of a {\phi} Josephson junction
We demonstrate experimentally the existence of Josephson junctions having a
doubly degenerate ground state with an average Josephson phase \psi=\pm{\phi}.
The value of {\phi} can be chosen by design in the interval 0<{\phi}<\pi. The
junctions used in our experiments are fabricated as 0-{\pi} Josephson junctions
of moderate normalized length with asymmetric 0 and {\pi} regions. We show that
(a) these {\phi} Josephson junctions have two critical currents, corresponding
to the escape of the phase {\psi} from -{\phi} and +{\phi} states; (b) the
phase {\psi} can be set to a particular state by tuning an external magnetic
field or (c) by using a proper bias current sweep sequence. The experimental
observations are in agreement with previous theoretical predictions
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
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.
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