What chemistry is there in high-temperature superconductivity? II. Why these mixed-valent copper oxides, and where else?

Abstract

A local-pair mechanism for high-temperature superconductivity in the quaternary copper oxide families based on (La/Sr)2CuO4 and YBa2Cu3O7 is developed. The available experimental data (as of 29 June 1987) are assessed particularly from a materials point of view, to make clear what the combination of features is in these systems that is so unusual as effectively to restrict high-temperature superconductivity to the mixed-valent copper oxides-and then only to certain of these. All the principal enabling factors ensue from the position of copper in the periodic table, and the potential core-like stability of the d10 configuration. The latter permits monovalence to be added to di- and trivalence for this element, as for silver and gold, though it is in copper that all three valencies are most readily stabilised. The proposed mechanism does not involve disproportionation, but fluctuations on pre-existing 3+d8 sites to 1+d10, using electrons drawn from the d9 sites over which the superconductivity is dominantly mediated. The late position of copper in the transition-metal series secures heavy p/d hybridisation which leads to the absence of pair-breaking local moments being established both at the di- and trivalent sites. Support for the present negative-U model is extracted from the author's earlier work on the metal-insulator transition. A more magnetically oriented model like the resonant valence bond model which has recently associated with observations in d1 TiOCl, etc., does not really seem suited to the geometry and electronic conditions in the present copper oxides. However it is suggested that TiB2, La3S4 and the Chevrel phases might be open to treatment by such a model. Comment is also made about how the superconductivity of PdHx and Ba(Pb/Bi)O3 might be seen in the light of the present model, and attention is directed once again to what might be happening in pressure-quenched CuCl and CdS.