Phase transitions in high-Tcsuperconductive oxides

Abstract

Abstract Superconductivity is stabilized by the opening up of an energy gap at the Fermi Energy E F below the superconductor transition temperature T c ; any other mechanism that opens an energy gap at E F is competitive. In the high-T c bismuth oxides, a charge-density wave (CDW) is competitive; in the high-T c copper oxides the correlation splitting associated with localized electrons is competitive. In each class of oxide, stabilization of superconductivity requires the introduction of mixed valency. It is argued that suppression of a CDW or of localized magnetic moments and long-range magnetic order by the introduction of mixed valency is not a sufficient condition for high-T c superconductivity. A common feature of the superconductive phase in all the p-type high-T c oxides is a Fermi energy that cuts two bands, a [sgrave]∗ band of primarily cationic character (Bi-6s or Cu-3dx2-y2) and a π or π∗ band of primarily anionic character (O-2pπ). The transition from localized copper magnetic moments to Pauli paramagnetism is monitored in the system La2-xSrxCuO4-y; although short-range antiferromagnetic order is retained in the p-type superconductive phase of the copper oxides, a charge-transfer as against a spin-spin exchange interaction appears to be the more probable mechanism for enhancement of the Cooper pairing below T c . The recently discovered n-type superconductivity in Nd2-xCexCuO4 may involve charge transport to the rare-earth 5d bands.