Van Hove Excitons and High-T$_c$ Superconductivity: VIIIC Dynamic Jahn-Teller Effects vs Spin-Orbit Coupling in the LTO Phase of La$_{2-x}$Sr$_x$CuO$_4$

TL;DR

This paper explores how van Hove singularities influence structural phase transitions in La$_{2-x}$Sr$_x$CuO$_4$, focusing on dynamic Jahn-Teller effects and their competition with spin-orbit coupling.

Contribution

It introduces a model Hamiltonian that describes phase transitions driven by dynamic Jahn-Teller effects in LSCO, highlighting the conditions for various JT phases.

Findings

Van Hove singularities can induce structural distortions via spin-orbit or Jahn-Teller effects.

The model predicts a sequence of phase transitions from disordered to dynamic and static Jahn-Teller phases.

Static Jahn-Teller phase may be absent depending on parameters.

Abstract

The possible role of the van Hove singularity (vHs) in stabilizing the low-temperature orthorhombic (LTO) phase transition in La$_{2-x}$\-Sr$_x$\-CuO$_ 4$ (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.