Van Hove exciton-cageons and high-Tc superconductivity

Abstract

Abstract The low-temperature orthorhombic (LTO) phase transition in La 2− x Sr x CuO 4 has been interpreted as a dynamic Jahn-Teller (JT) effect, with an octahedral tilt splitting the degeneracy of the two van Hove singularities. An explicit model of the LTO phase has been constructed as a soliton lattice: an array of domain walls separating regions of local low-temperature tetragonal (LTT) symmetry, but with a macroscopic average orthorhombic symmetry (‘orthons’). In this paper, the band structure is calculated for the simplest commensurate orthon superlattices, and it is shown that the degeneracy of the two van Hove singularities is broken in this phase. A similar splitting of the peak in the density of states (dos) is found for a CPA calculation of an arbitrary dynamic JT state. This splitting is essential to an understanding of the competition between structural instability and superconductivity in these materials. A third (unsplit) dos peak is also found, suggesting that the present model also provides a basis for explaining nanoscale phase disorder in these materials.