Correlation effects and superconductivity in cuprates: A critical account

Book chapter, 2011

Electronic structure of transition metal oxides and salts is reviewed, particularly what concerns electron transfer, electron correlation, electron-nuclear coupling, and inter-metal interaction in cuprates. A short resume of electronic correlation in molecular systems is first given. Electron pair transfer is treated in a many-electron real space approach using standard mixed-valence theories. The latter models have been successful in the past in describing the response of the electrons to the motion of the nuclei (electron - phonon interaction). The possibility for multiple oxidation states of the metal ion is typical for transition metal compounds. Mott-Hubbard-U is strictly defined, its dependence on breathing mode coordinates analysed, and the connection between U and the energy gap for superconductivity clarified. Delocalization is treated in terms of an extended Hush model and electron pair supercurrents derived at the Van Hove degeneracy. 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) can be connected to mixed-valence. 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. Finally comments are made on why MV-2 systems delocalize as Fermi- Dirac systems (ordinary metals) while MV-3 systems delocalize and condense as Bose-Einstein systems (superconductors).