A spin polaron model of high- Tc superconductivity

Abstract

Spin-polaron model has been studies as a basis for high- T c superconductivity. Parameterized antiferromagnetic band calculations for a CuO 2 plane show how a Mott-Hubbard (M-H) gap opens as a consequence of an electron repulsion energy U at the Cu sites. The parameters are chosen to give good agreement with measured density of states and the magnetic moment per Cu site. Variation of the hole concentration by Sr doping in La 2− x Sr x CuO 4 and O depletion in YBa 2Cu 3O 7− x moves the Fermi level relative to the M-H band edge. The holes induce spin deviations on the neighboring Cu sites to form small spin polarons, with effective masses several times the band masses. Two spin polarons have an attractive interaction because the spin deviations are partially “healed” when the polarons approach one another. The proximity of the Fermi level to the M-H band edge and the interplay of O 2pσ and 2pπ bands can provide fits to the variations of T c with x in La 2− x Sr x CuO 4 and YBa 2Cu 3O 7− x . A discussion of various aspects and implications of the model is given.