Unconventional superconducting properties of the high Tc oxides

Abstract

Some of the properties of the new high T c oxides are quite different from those of the conventional low T c superconducting metals and alloys. Most prominent is the low critical current density of polycrystalline samples, one of the outstanding problems that need to be solved for large scale applications. There is experimental evidence that grain boundaries, even if free of second-phase material, act as weak coupling junctions between grains rather than as pinning centres. On the other hand, very high critical current densities have been achieved in high quality oriented films and single crystals. There are also many indications that the oxides are basically BCS (Bardeen-Cooper-Schrieffer) superconductors, with a gap over the entire Fermi surface. We relate their unconventional properties to their short coherence length, rather than to a radically different type of superconductivity. This short coherence length renders the superconducting order parameter very sensitive to defects on the atomic scale. A high defect density, such as is observed in many of the oxides, drives them into an inhomogeneous regime, which we propose is the reason for the peculiar behaviour of the upper critical field, inter- and intragrain junctions, and in general weak superconductivity in high fields.