Nature of high-temperature superconductivity

Abstract

Evidence is presented that the superconducting hole condensate generally does not reside in the cuprate planes of high-temperature superconductors, but in the SrO layers, in the BaO layers, or in the regions of interstitial oxygen. Evidence that electrons, not holes, transfer to the cuprate planes of HgBa2Can−1CunO2+n+δ as a function of pressure, number n of layers, and increasing Tc is presented; holes transfer to the BaO layers. The hole transfer in YBa2Cu3O7 is also to the BaO layers. PrBa2Cu3O7 superconducts (as predicted) when it is free of pair-breaking PrBa defects in its BaO layers. The chosen locus of the superconductivity is consistent with the observation of magnetism in both the CuO layers and the cuprate planes of YBa2Cu3O7. Four materials were successfully predicted to superconduct by assuming that the cuprate planes are normal. There are no n-type high-temperature superconductors; Nd2−zCezCuO4 is p type and doped with interstitial oxygen. When Y+3 is replaced by Am+4, Pb2Sr2YCu3O8 becomes n type and stops superconducting. Holes remain near interstitial oxygen in Tl2Ba2Can−1CunO2n+4+δ. Gd2−zCezCuO4, unlike Nd2−zCezCuO4, does not superconduct because Gd has L=0 and J≠0 and breaks Cooper pairs associated with its interstitial oxygen, but Gd2−zCezSr2Cu2NbO10 does superconduct (in its SrO layers). YBa2Cu3O7 exhibits bulk nodeless (s-wave) superconductivity. We argue that the superconductivity of YBa2Cu3O7 is representative of high-Tc superconductors. The pairing mechanism is electronic (not phononic) and associated with holes on certain oxygen ions (or sulfur ions, in the case of some organic superconductors). We explore a Bardeen-Cooper-Schrieffer-type formalism applied to cuprates, ruthenates, and other compounds.