The Ising, Small-Bipolaron Theory of Cuprate Superconductivity

THE ISING, SMALL-BIPOLARON THEORY OF CUPRATE SUPERCONDUCTIVITY 

The crystal structures (Section 8.2.2) and the phase-relations (Section 8.2.3) of the cuprate superconductors are considerably more complex than for the metallic superconductors. It is not surprising that, while there a common macrotheory, different microtheories are required. 

The parent (A2B0CU-O4) cuprates are antiferromagnetic semiconductors which, on doping to A2-xBxCu-04, become superconductors where x is called the doping parameter. The replacement of Nd3 by Ce4+ produces an electron superconductor 

Cuprate superconductors exhibit complicated phase diagrams which are functions of the doping parameter, x which controls the amount of the electron-transfer into or out of the cuprate plane. See for example Fig. 8.2. 

Note that the undoped cuprate is an antiferromagnet and that doping converts it into a superconductor. We have shown above that antiferromagnetic behavior arises from the z = +1 side of the extended Hiickel-Hubbard spectrum in Fig. 4.1. In the ISB theory the superconducting behavior comes from the z = -1 side of Fig. 4.1 just as does the freeon theory of ferromagnetism. 

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