Abstract

The current-carrying properties of type II superconductors are of considerable importance for both fundamental and applied studies of these technologically important materials. The critical current density, J _c, in particular, is regarded by engineers as the key parameter to the practical application of low- and high-temperature superconductors in a diversity of engineering applications, such as MRI, Maglev, energy storage devices and motors and generators. As a result, the optimization of J _c for the different superconducting materials fabricated in the form of wires and tapes, thin films and bulks has been the focus of extensive research over many decades. J _c is essentially a characteristic of the so-called mixed state in type II materials, in which normal and superconducting phases co-exist thermodynamically. Magnetic flux enters type II superconductors in the form of lines of magnetic flux quanta, equivalent to Faraday's lines of force. These thread current vortices that circle predominantly normal regions of the material and give rise to some extraordinary magnetic behaviour, driven entirely by the ability of the material to resist the motion of these flux lines.