Abstract

Type II superconductors with high upper critical fields H c2 have considerable potential for practical applications, including high magnetic field solenoids, permanent magnets and energy storage devices, as well as magnetic field detectors (SQUIDs) and superconducting components for electronics and communications. However, the interplay between electrical currents and lines of quantised magnetic flux (the “flux lines”, “vortex lines”, or simply “vortices” of Section A3.1) in superconducting materials results in a driving force that puts the latter in motion. Vortex motion leads to dissipation of energy, manifested as an electric potential within the material, which therefore can no longer be considered to be superconducting, i.e. its electrical resistance becomes non-zero (Bardeen and Stephen 1965). Preventing vortex motion up to a high critical current density j c is therefore essential. This is achieved through pinning of the vortex lines by imperfections of the material. Fortunately, such flux pinning is a general phenomenon in commonly produced materials. In the following sections, an overview is given of this interesting phenomenon and some related issues.