Correlation bag and high-Tcsuperconductivity

Abstract

From an analysis of the normal-state properties of the high-${\mathit{T}}_{\mathit{c}}$ copper oxide superconductors the following features can be established on the basis of single-particle theories: (1) a bandwidth W≊U>${\mathit{E}}_{\mathit{p}}$, where U is the on-site correlation energy and ${\mathit{E}}_{\mathit{p}}$ is the pair binding energy; (2) a W≊8ħ${\mathrm{\omega }}_{\mathit{R}}$ at the narrow-band limit for small-polaron versus itinerant-electron conduction in a mixed-valent system; (3) a W≊${\mathrm{\varepsilon }}_{\mathrm{\sigma }}$ ${\lambda }_{\mathrm{\sigma }}^2$, where the covalent mixing parameter ${\lambda }_{\mathrm{\sigma }}$ varies sensitively with the hole concentration and consequently with any local charge fluctuations in a mixed-valent system; (4) on-site and near-neighbor correlation energies that vary sensitively with the bandwidth in the region W≊U. From the superconductor properties, a ξ≊10 Å signals an energy range of perturbed states ħω≲W, and superconductive pairs constrained to a small volume in real space which makes necessary the introduction of a nonretarded potential. These features lead us to consider charge fluctuations, induced by strong electron-lattice interactions, where U≳W≊8ħ${\mathrm{\omega }}_{\mathit{R}}$,