Zero-field incommensurate spin-Peierls phase with interchain frustration in TiOCl

TL;DR

This paper investigates TiOCl and TiOBr, revealing that interchain frustration leads to incommensurate and commensurate spin-Peierls phases driven by a single mechanism, challenging previous explanations involving orbital fluctuations.

Contribution

It demonstrates that interchain frustration causes incommensurate order in TiOCl and TiOBr, with a single spin-Peierls mechanism inducing multiple phase transitions, contrary to prior orbital fluctuation theories.

Findings

Interchain frustration causes incommensurate order.

Two phase transitions are driven by a single spin-Peierls mechanism.

Optical data rules out orbital fluctuations as the primary cause.

Abstract

We report on the magnetic, thermodynamic and optical properties of the quasi-one-dimensional quantum antiferromagnets TiOCl and TiOBr, which have been discussed as spin-Peierls compounds. The observed deviations from canonical spin-Peierls behavior, e.g. the existence of two distinct phase transitions, have been attributed previously to strong orbital fluctuations. This can be ruled out by our optical data of the orbital excitations. We show that the frustration of the interchain interactions in the bilayer structure gives rise to incommensurate order with a subsequent lock-in transition to a commensurate dimerized state. In this way, a single driving force, the spin-Peierls mechanism, induces two separate transitions.