Chemistry of high-temperature superconductors

The recent dramatic increases in superconducting Cu- and Bi-based oxides' T(c) have led to the perception that a totally new mechanism for this phenomenon is implicated. Guidelines have been developed for an understanding of the underlying mechanism on the basis of current observations and correlations. One such guideline is that superconductivity tends to occur at the boundary between localized and delocalized electron behavior; in addition, all oxide superconductors contain mixed-valent cations, and possess very high covalency which may be necessary for the development of bands able to support metallic behavior.