VANHOVE EXCITONS AND HIGH-T(C) SUPERCONDUCTIVITY .8C. DYNAMIC JAHN-TELLER EFFECTS VERSUS SPIN-ORBIT-COUPLING IN THE LTO PHASE OF LA2-XSRXCUO4

The possible role of the van Hove singularity (vHS) in stabilizing the low-temperature orthorhombic (LTO) phase transition in La2-xSrxCuO4 (LSCO) is discussed. It is found that the vHS can drive a structural distortion in two different ways, either due to spin-orbit coupling or to dynamic Jahn-Teller (JT) effects. This paper discusses the latter effect in some detail. It is shown that a model hamiltonian introduced earlier to describe the coupled electron-octahedral tilt motions (''cageons'') has a series of phase transitions, from a high-temperature disordered JT phase (similar to the high-temperature tetragonal phase of LSCO) to an intermediate temperature dynamic JT phase, of average orthorhombic symmetry (the LTO phase) to a low-temperature static JT phase (the low-temperature tetragonal phase). For some parameter values, the static JT phase is absent.