Optical study of MgTi$_2$O$_4$: Evidence for an orbital-Peierls state

TL;DR

This study uses optical measurements to investigate MgTi$_2$O$_4$, revealing evidence for an orbital-Peierls state that explains its metal-insulator transition through orbital ordering and one-dimensional Peierls instability.

Contribution

It provides experimental optical evidence supporting the orbital-Peierls state in MgTi$_2$O$_4$, linking orbital ordering to the metal-insulator transition in 3D spinel oxides.

Findings

Spectral changes across the transition align with 1D Peierls transition model.

Orbital ordering of $d_{yz}$ and $d_{zx}$ drives the transition.

Orbital-driven instability explains optical data in related compounds.

Abstract

Dimension reduction due to the orbital ordering has recently been proposed to explain the exotic charge, magnetic and structural transitions in some three-dimensional (3D) transitional metal oxides. We present optical measurement on a spinel compound MgTi$_2$O$_4$ which undergoes a sharp metal-insulator transition at 240 K, and show that the spectral change across the transition can be well understood from the proposed picture of 1D Peierls transition driven by the ordering of $d_{yz}$ and $d_{zx}$ orbitals. We further elaborate that the orbital-driven instability picture applies also very well to the optical data of another spinel CuIr$_2$S$_4$ reported earlier.