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.