Approaching Large U _d High-T _c Cuprates from the Covalent Side

Abstract

Large U _d theories of high-T _c cuprates often start from the ionic limit in which one charge per CuO₂ unit cell is localized on the copper site and involved in AF correlations with neighboring sites. AF correlations are promoted by a relatively small exchange J and the motion of holes by is described by the t–J models with narrow effective bands. However, very small hole doping is sufficient to destabilize the Mott-AF phase and the resulting metallic phase explicitly exhibits comparatively wide covalent bands. After a brief description of the large U _d ionic limit, the large U _d covalent limit is therefore considered. It is shown that in this limit, the large U _d produces a reasonably small effective kinematic interaction between two holes on oxygens. This interaction gives rise to magnetic correlations, similar to those found in the weak-coupling Hubbard theories. The distinct signature of large U _d is the broadening of the single (and two) particle properties of the system, which is related to the “mixed valence fluctuations” between Cu and 2O, localized within the CuO₂ unit cell. These fluctuations produce a sizeable Landau-like damping of the single (and two) particle propagations, thus competing with the magnetic coherence, characterized by an incommensurate wave-vector. The cuprates appear to fall between these two extreme limits, which might explain in part why they are so difficult to understand.