On the nature of antiferromagnetism in the CO_2 planes of oxide superconductors

Abstract

Recent results on electrons and holes doped CuO 2 planes confirm the marked covalency of CuO bonding, suggesting a band picture of long and short range antiferromagnetism. The maxima of superconductive T c versus doping can be related to the crossing by the Fermi level of the edges of the pseudogap due to antiferromagnetic short range order (bonding edge for holes doping, antibonding one for electrons doping). The symmetry of the superconductive gap can be related to the Bragg scattering of electronic Bloch states near the edges of the AF pseudogap. Assuming a standard phonon coupling, one then predicts for commensurate AF a pure d symmetry of the superconductive gap for underdoped samples and d symmetry plus an ip contribution increasing linearly with overdoping. This seems in agreement with recent measurements of gap symmetry for YBCO, but should be more fully tested, especially for electron doped samples. The simple band approximation used here could no doubt be made more realistic by a specific inclusion of electron correlations and by a better description of AF short range order. Uncommensurate AF, as in LSCO, is not considered here.