Electronic and phonon excitations in {\alpha}-RuCl$_3$

TL;DR

This study investigates the electronic and phonon excitations in { extalpha}-RuCl$_3$ across a broad frequency and temperature range, revealing a phase transition, a significant optical gap, and temperature-dependent spectral features.

Contribution

It provides comprehensive broadband optical spectra and insights into the phase transition, phonon behavior, and electronic gap in { extalpha}-RuCl$_3$, a candidate Kitaev material.

Findings

Significant reflectance change across the phase transition.

Identification of an optical energy gap around 200 meV.

Observation of a blue shift of the gap with increasing temperature.

Abstract

We report on THz, infrared reflectivity and transmission experiments for wave numbers from 10 to 8000 cm$^{-1}$ ($\sim$ 1 meV - 1 eV) and for temperatures from 5 to 295 K on the Kitaev candidate material {\alpha}-RuCl$_3$. As reported earlier, the compound under investigation passes through a first-order structural phase transition, from a monoclinic high-temperature to a rhombohedral low-temperature phase. The phase transition shows an extreme and unusual hysteretic behavior, which extends from 60 to 166 K. In passing this phase transition, in the complete frequency range investigated we found a significant reflectance change, which amounts almost a factor of two. We provide a broadband spectrum of dielectric constant, dielectric loss and optical conductivity from the THz to the mid infrared regime and study in detail the phonon response and the low-lying electronic density of states. We provide evidence for the onset of an optical energy gap, which is of order 200 meV, in good agreement with the gap derived from measurements of the DC electrical resistivity. Remarkably, the onset of the gap exhibits a strong blue shift on increasing temperatures.