Microscopic model for transitions from Mott to spin-Peierls insulator in TiOCl

TL;DR

This paper develops a microscopic model based on DFT calculations to explain the phase transitions in TiOCl, highlighting the role of magnetic frustration and phonons in the transition from Mott to spin-Peierls insulator.

Contribution

The paper introduces a detailed microscopic Hamiltonian for TiOCl, revealing the interplay of magnetic frustration and phonons in its phase transitions, which was not previously modeled.

Findings

Magnetic frustration competes with spin-Peierls distortion in TiOCl.

Adiabatic phonons trigger the spin-Peierls state.

The model explains the incommensurate phase in TiOCl.

Abstract

On the basis of ab initio density functional theory (DFT) calculations, we derive the underlying microscopic model Hamiltonian for TiOCl, a unique system that shows two consecutive phase transitions from a Mott insulator to a spin-Peierls insulator through a structurally incommensurate phase. We show with our model that the presence of magnetic frustration in TiOCl leads to a competition with the spin-Peierls distortion, which results in the novel incommensurate phase. In addition, our calculations indicate that the spin-Peierls state is triggered by adiabatic phonons, which is essential for understanding the nature of the phase transition.