CORRELATION BAG AND HIGH-TC SUPERCONDUCTIVITY

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 WU>Ep, where U is the on-site correlation energy and Ep is the pair binding energy; (2) a W 8Latin small letter h with strokeR at the narrow-band limit for small-polaron versus itinerant-electron conduction in a mixed-valent system; (3) a W 2, where the covalent mixing parameter 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 10 signals an energy range of perturbed states Latin small letter h with stroke 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 UW 8Latin small letter h with strokeR, 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. © 1990 The American Physical Society.