High temperature superconductivity; The spin polaron theory

An outline is given of one of the many models which have been advanced to explain high temperature superconductivity in the quasi-two-dimensional oxides. In these the compensation ensures that there are free positive holes, formed in the oxygen 2p states, which form ‘spin polarons’ by orienting neighbouring spins in Cu 3d⁹ ions. Two spin polarons then combine to form a spinless pair, which obeys Einstein–Bose statistics. It is shown that a degenerate gas of such entities is a superconductor. If this is a correct model, such pairs (bosons) must exist above the critical temperature (T _c). Particular attention is given to the properties of these materials at temperatures above T _c. An unresolved problem is whether all, or only some, of the carriers form bosons.