Superconductivity in Cuprates: Details of Electron Phonon Coupling
Journal article, 2010
The Bardeen-Cooper-Schrieffer (BCS) model explains superconductivity (SC) as due to correlation between electronic momentum and nuclear momentum (phonons) in a free electron gas. The BCS model lacks chemical specificity, however, since the coupling mechanism is left unspecified. After the discovery of high T-C superconductivity in 1986 it was concluded that electron-phonon interactions are insufficient to explain electron pairing. A large part of theoretical research has since been aiming at finding another mechanism that would allow us to consider the superconducting system as a gas of charged free bosons. However, there appears to be no reason to assume free electrons in oxides. In this article the free-electron criterion is therefore replaced by the criterion that a pair of electrons can move freely between sites without resistance, i.e., without activation energy. Electron pair transfer is treated in a many-electron real space approach using standard mixed-valence theories. Mott-Hubbard-U is strictly defined, its dependence on breathing mode coordinates analyzed, and the connection between U and the energy gap for superconductivity clarified. d-wave gap anisotropy is found to be consistent with the general atomic level model presented here. Softening of phonon half-breathing modes in inelastic neutron scattering (INS) is connected to mixed-valency. The fundamental vibronic interaction between spin density wave (SDW) and charge density wave (CDW) states leads to a new phase with energy gap and electron pair carriers that can only be the superconducting phase. (C) 2009 Wiley Periodicals, Inc. Int J Quantum Chem 110: 1177-1126, 2010.