Resistivity measurements on bulk Bi2Sr2CaCu2O8+δ: Contribution of vortices at low magnetic fields

Abstract

As expedient as high critical temperature superconductors (HTSC) are, their properties in the presence of an external magnetic field should inexorably be studied since these materials would necessarily be exposed to these fields in technological use.
An applied magnetic field penetrates a superconductor in the mixed state. The penetration occurs in the form of tubes, called vortices, which serve to confine the flux. The interaction between vortices is repulsive so that they arrange themselves as far away from each other, as in a flux lattice in clean samples or as vortex glass in the presence of randomly distributed pinning centers. The pinning centers may be point defects, grain boundary or intra/intergranular non-superconducting regions. On the other hand, transport currents and thermal fluctuations can both induce flux motion, due to the Lorentz force, and produce dissipation. This can involve, for example, the release and transportation of vortices to other pinning centers, and the pinning and depinning of flux bundles. Flux flow can be viewed as a rate of change of flux which, by Faraday's law, produces a voltage drop in the superconductor along the direction of the transport current. The resistance associated with this motion provides a mechanism for heat dissipation.
In this study, the contribution of these vortices were investigated through resistivity measurements in varying magnetic fields. This contribution was quantitatively determined as the flux flow resitivity.