engleski

Dynamic properties of vortices in type II superconductors

I shall present the results of a series of experiments pertaining to fundamental issues of vortex dynamics in type II superconductors. The experiments were carried out on samples particularly suitable for such a research, namely amorphous Nb0.7Ge0.3 thin films which have a simple, weakly pinned vortex matter and an experimentally accessible (B, T) range of the mixed state. A small driving force on vortices produces a weakly nonequilibrium situation in which the equilibrium vortex properties are preserved. The related power dissipation reflects either flux flow or depinning phenomena, the V(I) curve being linear or moderately nonlinear, respectively. At a large driving force the induced electric field in vortex cores is large enough to alter the spectrum of normal excitations. In this strongly nonequilibrium regime vortices undergo qualitative changes manifested by a change of the core area. Depending on the experimental conditions they either shrink or expand, in both cases leading to a nonlinear V(I) that is not related to the pinning force. The softness of the vortex-core boundary can as well be probed by noise measurements. The voltage noise in the depinned state is consistent with the vortex-core-size fluctuations and it is of a form characteristic of resistance fluctuations in electronic conduction.

Superconductivity; vortex

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