Strong electron correlation and phonon coupling in high Tc superconductors

Systems with one active electron (or hole) per site may be spin-alternating (SDW) or charge alternating (CDW). Typical SDW systems (CuO) cannot be described with a single Slater determinant. Electron correlation is also important in typical CDW systems such as BaBiO3 (on-site correlation). If the CDW and the SDW phases have similar free energies, they are either connected by a phase transition, or, if the zero-point vibrations overlap sufficiently, mix quantum mechanically and form a new phase. It will be shown, using very accurate quantum chemical calculations that a four-site Cu-O cluster has local energy minima for both phases. The properties of the mixed phase are those of a superconductor: The electrons are coupled in pairs, move in a delocalized non-activated way, and give a current in a magnetic field equivalent to the Meissner effect. (c) 2007 Elsevier B.V. All rights reserved.