Orbital order in the low-dimensional quantum spin system TiOCl probed by ESR

TL;DR

This study uses ESR to investigate orbital order in TiOCl, revealing a dominant d_{xy} orbital occupation, formation of spin chains, and a transition to a nonmagnetic state at 67 K, highlighting strong spin-lattice coupling.

Contribution

First ESR evidence of orbital order in TiOCl confirming theoretical predictions and linking orbital occupation to spin chain formation and phase transition.

Findings

Dominant d_{xy} orbital occupation confirmed by ESR

Formation of linear spin chains along the b axis

Transition to nonmagnetic ground state at 67 K

Abstract

We present electron spin resonance data of Ti$^{3+}$ (3$d^1$) ions in single crystals of the novel layered quantum spin magnet TiOCl. The analysis of the g tensor yields direct evidence that the d_{xy} orbital from the t_{2g} set is predominantly occupied and owing to the occurrence of orbital order a linear spin chain forms along the crystallographic b axis. This result corroborates recent theoretical LDA+U calculations of the band structure. The temperature dependence of the parameters of the resonance signal suggests a strong coupling between spin and lattice degrees of freedom and gives evidence for a transition to a nonmagnetic ground state at 67 K.