Correlation bag and high-Tc superconductivity

Abstract

From an analysis of the normal-state properties of the high-Tc copper oxide superconductors the following features can be established on the basis of single-particle theories: (1) a bandwidth W≊U>Ep, where U is the on-site correlation energy and Ep is the pair binding energy; (2) a W≊8ħωR at the narrow-band limit for small-polaron versus itinerant-electron conduction in a mixed-valent system; (3) a W≊ɛσλσ2, where the covalent mixing parameter λσ 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≊8ħωR, in which ‘‘bags’’ rich in charge carriers coexist with regions poor in charge carriers together with an important modulation of the bandwidth—and hence the correlation energies—on moving from outside to inside a bag. The problems with the Alexandrov model that have been raised by De Jongh are resolved by the modulation of the correlation energies, which adds a new term to the Hamiltonian that gives an additional component to the binding energy. A bag model allows use of the spin-bag formalism but with a renormalization of the charge channel rather than the spin channel to obtain a possible solution of the new Hamiltonian. The model allows interpretation of the variation of Tc with hole concentration in the p-type copper oxides as well as the pressure dependence of Tc as a function of hole concentration. A bending of the Cu-O-Cu bond angles from 180° and the degree of freedom of the Cu-O bond length normal to the CuO2 sheets allows identification of possible vibrational modes involved in bag formation.

Source-reported materials — not catalogue approval

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