Abstract

The spin-polaron concept is introduced in analogy to ionic and electronic polarons and the assumptions underlying the author's approach to spinpolaron-mediated high-Tc superconductivity are discussed. Elementary considerations about the spin-polaron formation energy are reviewed and the possible origin of the pairing mechanism illustrated schematically. The electronic structure of the CuO2 planes is treated from the standpoint of antiferromagnetic band calculations that lead directly to the picture of holes predominantly on the oxygen sublattice in a Mott–Hubbard/charge transfer insulator. Assuming the holes to be described in a Bloch representation but with the effective mass renormalized by spin-polaron formation, equations for the superconducting gap, Δ, and transition temperature, Tc, are developed and the symmetry of Δ discussed. After further simplifications, Tc is calculated as a function of the carrier concentration, x. It is shown that the calculated behavior of Tc(x) follows the experimental results closely and leads to a natural explanation of the effects of under- and over-doping. The paper concludes with a few remarks about the evidence for the carriers being fermions (polarons) or bosons (bipolarons).

Introduction

A carrier (electron or hole) moving through an ionic lattice will induce displacements of the ions and, under certain conditions, the carrier plus ionic displacements may form a good quasi-particle, i.e., the ionic polaron [1]. Similarly, electronic polarons may form when the carriers induce polarization of localized or quasi-localized electronic distributions. In an analagous manner, a spin polaron is a spin ½ carrier moving in a magnetic medium accompanied by deviations of localized ionic spins.