Relevance of nonadiabatic effects in TiOCl

TL;DR

This paper investigates the role of nonadiabatic phonon effects in TiOCl, showing that significant spin-phonon coupling is necessary to explain experimental observations of the incommensurate transition and spin gap.

Contribution

It introduces a nonadiabatic formalism for spin-Peierls systems and applies it to TiOCl, revealing the importance of nonadiabatic effects in modeling its phase transition.

Findings

High spin-phonon coupling needed to match the low-temperature spin gap.

The theory explains the incommensurate transition temperature and wave vector.

Renormalization of phonons aligns with inelastic x-ray experiments.

Abstract

We analyze the effect of the phonon dynamics on a recently proposed model for the uniform-incommensurate transition seen in TiOX compounds. The study is based on a recently developed formalism for nonadiabatic spin-Peierls systems based on bosonization and a mean field RPA approximation for the interchain coupling. To reproduce the measured low temperature spin gap, a spin-phonon coupling quite bigger than the one predicted from an adiabatic approach is required. This high value is compatible with the renormalization of the phonons in the high temperature phase seen in inelastic x-ray experiments. Our theory accounts for the temperature of the incommensurate transition and the value of the incommensurate wave vector at the transition point.