Raman evidence for dimerization and Mott collapse in $\alpha$-RuCl$_3$ under pressures

TL;DR

This study uses Raman spectroscopy to investigate pressure-induced phase transitions in $ ext{α-RuCl}_3$, revealing stacking order changes, in-plane Ru-Ru dimerization, and a Mott insulator collapse, highlighting complex interactions in Kitaev materials.

Contribution

First demonstration of pressure-induced in-plane Ru-Ru dimerization and Mott collapse in $ ext{α-RuCl}_3$ using Raman spectroscopy, elucidating competing interactions in Kitaev compounds.

Findings

Identification of two critical pressures at 1.1 GPa and 1.7 GPa.

Observation of stacking order transition at 1.1 GPa.

Evidence of Ru-Ru bond dimerization at 1.7 GPa.

Abstract

We perform Raman spectroscopy studies on $\alpha$-RuCl$_3$ at room temperature to explore its phase transitions of magnetism and chemical bonding under pressures. The Raman measurements resolve two critical pressures, about $p_1=1.1$~GPa and $p_2=1.7$~GPa, involving very different intertwining behaviors between the structural and magnetic excitations. With increasing pressures, a stacking order phase transition of $\alpha$-RuCl$_3$ layers develops at $p_1=1.1$~GPa, indicated by the new Raman phonon modes and the modest Raman magnetic susceptibility adjustment. The abnormal softening and splitting of the Ru in-plane Raman mode provide direct evidence of the in-plane dimerization of the Ru-Ru bonds at $p_2=1.7$~GPa. The Raman susceptibility is greatly enhanced with pressure increasing and sharply suppressed after the dimerization. We propose that the system undergoes Mott collapse at $p_2=1.7$~GPa and turns into a dimerized correlated band insulator. Our studies demonstrate competitions between Kitaev physics, magnetism, and chemical bondings in Kitaev compounds.