12 research outputs found
Evidence for Thermally Activated Spontaneous Fluxoid Formation in Superconducting Thin-Film Rings
We have observed spontaneous fluxoid generation in thin-film rings of the
amorphous superconductor MoSi, cooled through the normal-superconducting
transition, as a function of quench rate and externally applied magnetic field,
using a variable sample temperature scanning SQUID microscope. Our results can
be explained using a model of freezout of thermally activated fluxoids,
mediated by the transport of bulk vortices across the ring walls. This
mechanism is complementary to a mechanism proposed by Kibble and Zurek, which
only relies on causality to produce a freezout of order parameter fluctuations.Comment: 4 pages, 3 figure
Antivortices due to competing orbital and paramagnetic pair-breaking effects
Thermodynamically stable vortex-antivortex structures in a
quasi-two-dimensional superconductor in a tilted magnetic field are predicted.
For this geometry, both orbital and spin pair-breaking effects exist, with
their relative strength depending on the tilt angle \Theta. The spectrum of
possible states contains as limits the ordinary vortex state (for large \Theta)
and the Fulde-Ferrell-Larkin-Ovchinnikov state (for \Theta=0). The
quasiclassical equations are solved near H_{c2} for arbitrary \Theta and it is
shown that stable states with coexisting vortices and antivortices exist in a
small interval close to \Theta=0. The results are compared with recent
predictions of antivortices in mesoscopic samples.Comment: 11 pages, 3 figure
Fluxoid dynamics in superconducting thin film rings
We have measured the dynamics of individual magnetic fluxoids entering and
leaving photolithographically patterned thin film rings of the underdoped
high-temperature superconductor BiSrCaCuO, using a
variable sample temperature scanning SQUID microscope. These results can be
qualitatively described using a model in which the fluxoid number changes by
thermally activated nucleation of a Pearl vortex in, and transport of the Pearl
vortex across, the ring wall.Comment: 9 pages, 10 figures, fixed typo
Search for Spontaneous Nucleation of Magnetic Flux During Rapid Cooling of YBCO films Through Tc
We describe an experimental search for spontaneous formation of flux lines
during a rapid quench of thin YBaCuO films through Tc. This effect is expected
according to the Kibble-Zurek mechanism of a creation of topological defects of
the order parameter during a symmetry breaking phase transition. Spontaneously
formed vortices were previously observed in superfluid 3He, while a similar
experiment in superfluid 4He gave negative results. Using a high Tc SQUID, we
measured both the magnetic flux in the sample during a quench with a
sensitivity of 20 phi-0/cm^2, and the field noise which one would expect from
flux lines pinned in the film. The sensitivity was sufficient to detect
spontaneous flux at a level corresponding to 10^(-3) of the prediction. Within
our resolution, we saw no evidence for this effect.Comment: Manuscript and 4 figure
Spontaneous magnetic-flux generation in superconducting ring
The Kibble-Zurek scenario for nucleation of topological defects of the order
parameter during a symmetry-breaking phase transition in a superconducting
ring is investigated. Solving numerically the set of time-dependent
Ginzburg-Landau (TDGL) equations coupled with the temperature
diffusion equation, we predict spontaneous generation of the magnetic flux
inside the superconducting ring. Josephson weak links in the ring increase
the magnitude of the spontaneously generated magnetic flux
Relaxation of normal spot in type-II superconductors
Relaxational dynamics of the non-equilibrium normal domain in a
superconductor is considered. It is shown, both analytically and numerically,
that this relaxation leads to nucleation of topological vortices and
antivortices. In the presence of strong pinning such a vortex/antivortex
phase persists infinitely