Depinning at the initial stage of the resistive transition in superconductors with a fractal cluster structure

Abstract

Depinning of vortices in percolative superconductor containing fractal clusters of a normal phase is considered. Transition of the superconductor into a resistive state corresponds to the percolation transition from a pinned vortex state to a resistive state when the vortices are free to move. The motion of the magnetic flux transferred by these vortices gives rise to the region of initial dissipation on current-voltage (U-I) characteristic. The influence of normal phase clusters on distinctive features of U-I characteristics of percolative type-II superconductors is considered. It is found that an increase in the fractal dimension of the normal phase clusters causes the initial dissipation region to broaden out. The reason of this effect is an increase in the density of free vortices broken away from the pinning centers by the Lorentz force. Dependencies of the free vortex density on the fractal dimension of the normal phase cluster boundaries are obtained.The dynamics of vortices in superconductors with fractal boundaries between the normal and superconducting phases has recently received much attention [1] - [3]. The study of their U-I characteristics enable to get new information on the electromagnetic properties as well as on the nature of a vortex state in such materials. New problems encountered in this field are of interest in view of their importance for the application of superconducting composites in electronics and power engineering, especially, for superconducting wire fabrication. Superconductors containing fractal clusters have specific magnetic and transport properties [4], [5]. The possibility to increase the critical current through the enhancement of pinning by the fractal normal phase clusters is of a particular interest [6], [7]. This paper is devoted to an analysis of the initial region of the resistive transition where the energy dissipation setsin. Here the process of vortex depinning gradually accrues resulting finally in the destruction of superconducting state because of the thermo-magnetic instability. The problem of initial dissipation in high-temperature superconductors (HTS's) has been studied by many authors [8] Let us consider a superconductor containing inclusions of a normal phase, which are out of contact with one another. We will suppose that the characteristic sizes of these inclusions far exceed both the superconducting coherence length and the penetration depth. A prototype of such a structure is a superconducting wire.The first generation HTS wires are fabricated following the powder-in-tube technique (PIT). The metal tube is being filled with HTS powder, then the thermal and deformation treatment is being carried out. The resulting product is the wire (tape) consisting of one or more superconducting cores armored by the normal metal sheath. The sheath endows the wire with the necessary mechanical (flexibility, folding strength) and electrical (the possibility to 1 release an excessive power when the superconductivity will be suddenly lost) properties. At the present time...

Fractal Boundaries of Vortex Pinning Clusters in Copper-oxide Superconductors in Magnetic Field

“…The peculiarities of the normal phase clusters were considered in detail in papers [12][13][14][15]. In the article [8] it was found that the DC magnetic field strongly influences on the fractal dimension of the boundaries of the normal phase clusters.…”