VANHOVE EXCITONS AND HIGH-T(C) SUPERCONDUCTIVITY .8A. VALENCE BOND DENSITY WAVES

It has recently been shown that an important role of correlation effects is to renormalize the band structure to be more 'Hubbard-like' at half-filling, driving a Mott transition. Using this insight, the density wave (DW)-superconductivity competition in the high-T(c) oxides is reexamined. It is found that both the low-temperature orthorhombic (LTO) and the low-temperature tetragonal (LTT) transitions in La2-xBaxCuO4 (LBCO) and related compounds are driven by the strong nesting of the van Hove singularity - the LTT as a static and the LTO a dynamic Jahn-Teller effect. A mean-field theory of the LTO phase is presented, in terms of a coherent superposition of charge DWs or a 'valence bond density wave'. The resulting DW-superconductivity phase diagram strongly resembles that found in LBCO. Connections of the present model to the magnetic pseudogap and to one-dimensionality are discussed.