PHENOMENOLOGY OF THE VALENCE FLUCTUATION PAIRING MECHANISM - T-C SYSTEMATICS OF CUPRATES, AND EVIDENCE FOR THIS MECHANISM IN OTHER SUPERCONDUCTORS

The phenomenology of the cuprate superconductors is unconventional in many respects, in addition to the high T-c's. A number of these features have recently been explained, qualitatively and even semi-quantitatively, by means of a valence-fluctuation mechanism. We now examine the extent to which this mechanism can account, at a qualitative level, for the T-c systematics of the many known cuprate superconductors. We find that this mechanism is consistent with some major features of the T-c systematics: the strong dependence on the hole-doping concentration x, the strong correlation with the Madelung potential of the apical oxygens, and a strong dependence on the number of apical oxygens per planar copper ion. We also argue that the same mechanism should be operating (together with phonons) in a number of other superconductors, in particular in the other ''exotic'' superconductors of Uemura. The evidence is that typical features of these exotics include extremely short coherence length, very high (and often T-2) normal-state resistivity, a highly anomalous form of H-c2(T), a large London penetration depth, and an often obvious suitability for representation by an Anderson lattice form of model Hamiltonian. The frequently-encountered cluster and/or reduced-dimensionality aspects are argued to be features which help to justify the latter form of model Hamiltonian.