634 research outputs found
Spontaneous Fluxon Formation in Annular Josephson Tunnel Junctions
It has been argued by Zurek and Kibble that the likelihood of producing
defects in a continuous phase transition depends in a characteristic way on the
quench rate. In this paper we discuss our experiment for measuring the
Zurek-Kibble scaling exponent sigma for the production of fluxons in annular
symmetric Josephson Tunnel Junctions. The predicted exponent is sigma = 0.25,
and we find sigma = 0.27 +/- 0.05. Further, there is agreement with the ZK
prediction for the overall normalisation
A quantitative investigation of the effect of a close-fitting superconducting shield on the coil-factor of a solenoid
Superconducting shields are commonly used to suppress external magnetic
interference. We show, that an error of almost an order of magnitude can occur
in the coil-factor in realistic configurations of the solenoid and the shield.
The reason is that the coil-factor is determined by not only the geometry of
the solenoid, but also the nearby magnetic environment. This has important
consequences for many cryogenic experiments involving magnetic fields such as
the determination of the parameters of Josephson junctions, as well as other
superconducting devices. It is proposed to solve the problem by inserting a
thin sheet of high-permeability material, and the result numerically tested.Comment: 3 pages, 4 figures, submitted to AP
Spontaneous Fluxoid Formation in Superconducting Loops
We report on the first experimental verification of the Zurek-Kibble scenario
in an isolated superconducting ring over a wide parameter range. The
probability of creating a single flux quantum spontaneously during the fast
normal-superconducting phase transition of a wide Nb loop clearly follows an
allometric dependence on the quenching time , as one would expect if
the transition took place as fast as causality permits. However, the observed
Zurek-Kibble scaling exponent is two times larger than
anticipated for large loops. Assuming Gaussian winding number densities we show
that this doubling is well-founded for small annuli.Comment: 10 pages, 2 figures, submitted to Phys. Rev. Lett
Gaussianity revisited: Exploring the Kibble-Zurek mechanism with superconducting rings
In this paper we use spontaneous flux production in annular superconductors
to shed light on the Kibble-Zurek scenario. In particular, we examine the
effects of finite size and external fields, neither of which is directly
amenable to the KZ analysis. Supported by 1D and 3D simulations, the properties
of a superconducting ring are seen to be well represented by analytic Gaussian
approximations which encode the KZ scales indirectly. Experimental results for
annuli in the presence of external fields corroborate these findings.Comment: 20 pages, 10 figures; submitted to J. Phys: Condens. Matter for the
special issue 'Condensed Matter Analogues of Cosmology'; v2: considerably
reduced length, incorporation of experimental details into main text,
discussion improved, references added, version accepted for publicatio
Planar Josephson Tunnel Junctions in a Transverse Magnetic Field
Traditionally, since the discovery of the Josephson effect in 1962, the
magnetic diffraction pattern of planar Josephson tunnel junctions has been
recorded with the field applied in the plane of the junction. Here we discuss
the static junction properties in a transverse magnetic field where
demagnetization effects imposed by the junction geometry and configuration of
the electrodes are important. Measurements of the critical current versus
magnetic field in planar Nb-based high-quality junctions with different
geometry, size and critical current density show that it is advantageous to use
a transverse magnetic field rather than an in-plane field to suppress the
Josephson tunnel current and Fiske resonances in practical applications.Comment: 5 pages, 2 figures, submitted to Journal of Applied Physic
Zurek-Kibble Mechanism for the Spontaneous Vortex Formation in Josephson Tunnel Junctions: New Theory and Experiment
New scaling behavior has been both predicted and observed in the spontaneous
production of fluxons in quenched annular Josephson tunnel
junctions as a function of the quench time, . The probability
to trap a single defect during the N-S phase transition clearly follows an
allometric dependence on with a scaling exponent , as
predicted from the Zurek-Kibble mechanism for {\it realistic} JTJs formed by
strongly coupled superconductors. This definitive experiment replaces one
reported by us earlier, in which an idealised model was used that predicted
, commensurate with the then much poorer data. Our experiment
remains the only condensed matter experiment to date to have measured a scaling
exponent with any reliability.Comment: Four pages, one figur
Spontaneous Fluxon Production in Annular Josephson Tunnel Junctions in the Presence of a Magnetic Field
We report on the spontaneous production of fluxons in the presence of a
symmetry-breaking magnetic field for annular Josephson tunnel junctions during
a thermal quench. The dependence on field intensity of the probability
to trap a single defect during the N-S phase transition drastically
depends on the sample circumferences. We show that the data can be understood
in the framework of the Kibble-Zurek picture of spontaneous defect formation
controlled by causal bounds.Comment: Submitted to Phys. Rev. B with 5 figures on Nov. 15, 200
-biased Josephson tunnel junctions
The behavior of a long Josephson tunnel junction drastically depends on the
distribution of the dc bias current. We investigate the case in which the bias
current is fed in the central point of a one-dimensional junction. Such
junction configuration has been recently used to detect the persistent currents
circulating in a superconducting loop. Analytical and numerical results
indicate that the presence of fractional vortices leads to remarkable
differences from the conventional case of uniformly distributed dc bias
current. The theoretical findings are supported by detailed measurements on a
number of -biased samples having different electrical and geometrical
parameters.Comment: 14 pages, 10 figure
Long Josephson Tunnel Junctions with Doubly Connected Electrodes
In order to mimic the phase changes in the primordial Big Bang, several
"cosmological" solid-state experiments have been conceived, during the last
decade, to investigate the spontaneous symmetry breaking in superconductors and
superfluids cooled through their transition temperature. In one of such
experiments the number of magnetic flux quanta spontaneously trapped in a
superconducting loop was measured by means of a long Josephson tunnel junction
built on top of the loop itself. We have analyzed this system and found a
number of interesting features not occurring in the conventional case with
simply connected electrodes. In particular, the fluxoid quantization results in
a frustration of the Josephson phase, which, in turn, reduces the junction
critical current. Further, the possible stable states of the system are
obtained by a self-consistent application of the principle of minimum energy.Comment: 34 pages, 9 figures, Phys. Rev. B April 201
New Experiments for Spontaneous Vortex Formation in Josephson Tunnel Junctions
It has been argued by Zurek and Kibble that the likelihood of producing
defects in a continuous phase transition depends in a characteristic way on the
quench rate. In this paper we discuss an improved experiment for measuring the
Zurek-Kibble scaling exponent for the production of fluxons in
annular symmetric Josephson Tunnel Junctions. We find .
Further, we report accurate measurements of the junction gap voltage
temperature dependence which allow for precise monitoring of the fast
temperature variations during the quench.Comment: 12 pages, 5 figures, submitted to Phys. Rev.
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