POLARON MODELS OF HIGH-TEMPERATURE SUPERCONDUCTIVITY

A review is given of theories of high-temperature superconductors in which the current is carried by bipolarons, which form a condensed Bose ps below T(c) and a non-degenerate ps above it. Such theories were first proposed by Schafroth, Alexandrov, Ranninger and de Jongh; the present author has, for the copper oxide materials, proposed spin bipolarons. Experimental work has, however, shown no magnetic moments in the superconducting and ''spin glass'' ranges of composition; a modification of the spin bipolaron model is proposed to take account of these observations. Other aspects of the model are discussed, particularly heat conduction and the effect of disorder. A comparison is made with the cubic bismuth materials.