Characteristic features of the exotic superconductors

Abstract

The exotic superconductors of this survey are those defined by Uemura and co-workers — the materials which approximately satisfy an empirical relation T c δ λ L −2, where λ L is the London penetration depth. As superconductors these materials are strange in many respects — in their chemistry and crystal structures, in many of their electronic and superconducting properties, and in their often conspicuously high transition temperatures. This category includes all of the presently known high-temperature superconductors. We now examine their unusual features in considerable detail, to sort out the features which are apparently universal (such as strong type-II behavior and high resistivity), or which are non-universal but common enough to be considered typical for this class (such as gap nodes). Several characteristics of the crystal chemistry are identified. Although fragments of this program have been reported often, this is the first attempt at a comprehensive examination. There appears to be a quasi-continuum of electronic behaviors, ranging from strongly exotic down to barely exotic cases. The location of a material within this continuum may thus depend on the relative strength of some “new” mechanism, as compared to the conventional phonon mechanism. In various places this survey is broadened to include other “strange formula” superconductors, since many other materials share the typical crystal-chemistry features of the exotic materials.