Microscopic Concepts of Superconductivity

In part I the transition temperature Tc for conventional superconductors is calculated; Bardeen, Cooper, Schrieffer (BCS) –, Eliashberg –, and Berk, Schrieffer – theories are compared. The Schrieffer-Wolff transformation, the Bogoliubov transformation, and the Green-function technique are applied as methods. In part II models for high-Tc superconductivity (HTSC) in cuprates are investigated. In 1986 HTSC for La2−x Bax CuO4 , with Tc = 30 K at x = 0.2, was demonstrated for the first time by Bednorz and Müller. Based on the experimental background up to 1992 the Emery model and the Anderson-lattice model play a fundamental role at the theoretical description. The system of itinerant O(p) holes and localized Cu(d) spins is used to deduce the so-called t–J or p–d models. Here the tpp − Jpd − Jdd model is derived. There appears a competition between antiferromagnetism and possible HTSC.