336 research outputs found
Self-Organized Criticality Effect on Stability: Magneto-Thermal Oscillations in a Granular YBCO Superconductor
We show that the self-organized criticality of the Bean's state in each of
the grains of a granular superconductor results in magneto-thermal oscillations
preceding a series of subsequent flux jumps. We find that the frequency of
these oscillations is proportional to the external magnetic field sweep rate
and is inversely proportional to the square root of the heat capacity. We
demonstrate experimentally and theoretically the universality of this
dependence that is mainly influenced by the granularity of the superconductor.Comment: submitted to Physical Review Letters, 4 pages, RevTeX, 4 figures
available as uufile
An Intuitionistic Formula Hierarchy Based on High-School Identities
We revisit the notion of intuitionistic equivalence and formal proof
representations by adopting the view of formulas as exponential polynomials.
After observing that most of the invertible proof rules of intuitionistic
(minimal) propositional sequent calculi are formula (i.e. sequent) isomorphisms
corresponding to the high-school identities, we show that one can obtain a more
compact variant of a proof system, consisting of non-invertible proof rules
only, and where the invertible proof rules have been replaced by a formula
normalisation procedure.
Moreover, for certain proof systems such as the G4ip sequent calculus of
Vorob'ev, Hudelmaier, and Dyckhoff, it is even possible to see all of the
non-invertible proof rules as strict inequalities between exponential
polynomials; a careful combinatorial treatment is given in order to establish
this fact.
Finally, we extend the exponential polynomial analogy to the first-order
quantifiers, showing that it gives rise to an intuitionistic hierarchy of
formulas, resembling the classical arithmetical hierarchy, and the first one
that classifies formulas while preserving isomorphism
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
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
Properties of superconducting MgB_2 wires: "in-situ" versus "ex-situ" reaction technique
We have fabricated a series of iron-sheathed superconducting wires prepared
by the powder-in-tube technique from (MgB_2)_{1-x}:(Mg+2B)_x initial powder
mixtures taken with different proportions, so that x varies from 0 to 1. It
turned out that "ex-situ" prepared wire (x = 0) has considerable disadvantages
compared to all the other wires in which "in-situ" assisted (0 < x < 1) or pure
"in-situ" (x = 1) preparation was used due to weaker inter-grain connectivity.
As a result, higher critical current densities J_c were measured over the
entire range of applied magnetic fields B_a for all the samples with x > 0.
Pinning of vortices in MgB_2 wires is shown to be due to grain boundaries.
J_c(B_a) behavior is governed by an interplay between the transparency of grain
boundaries and the amount of "pinning" grain boundaries. Differences between
thermo-magnetic flux-jump instabilities in the samples and a possible threat to
practical applications are also discussed.Comment: To be published in Supercond. Sci. Technol. (2003), in pres
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.
Proximity Effect Enhancement Induced by Roughness of SN Interface
Critical temperature reduction is considered for a thin film of
a layered superconductor (S) with a rough surface covered by a thick layer of a
normal metal (N). The roughness of the SN interface increases the penetration
of electrons from the normal metal into the superconductor and leads to an
enhancement of the proximity effect. The value of induced by the
roughness of the SN interface can be much higher than for a film
with a plain surface for an extremely anisotropic layered superconductor with
the coherence lengths .Comment: 2 page
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