The other high-temperature superconductors

Abstract

Since the discovery of the high-temperature superconducting cuprates in 1986, there have been four classes of superconductors discovered that have members with critical temperature higher than the highest known in 1986. These classes are typified by (in chronological order) the representatives: Ba 1− x K x BiO 3, Cs 3C 60, YPd 2B 2C, and Na x HfNCl, with maximum critical temperatures, respectively, of 35, 40, 23, and 25 K. These new superconductors are mostly different from the older superconductors, and certainly different from the layered cuprates. For several characteristics that are possibly important for superconductivity, two or three of the classes share similarities. There seems, however, to be almost no feature that is shared by all four classes. However, the three for which there is isotope shift data clearly qualify as phonon-coupled superconductors.