# Optical and photoelectrical studies on anisotropic metal-insulator transition of RuAs

## TL;DR

This study investigates the anisotropic electronic structure changes during the two-step metal-insulator transition in RuAs using optical conductivity, photoelectron spectra, and band calculations, revealing anisotropic gap formation without electron correlations.

## Contribution

It provides detailed insights into the anisotropic nature of the MIT in RuAs and clarifies the electronic structure in high and low-temperature phases through combined experimental and theoretical analysis.

## Key findings

- Energy gaps form anisotropically along different crystal axes.
- The gap opening temperature varies along the c and b axes.
- The two-step MIT is driven by anisotropic gap formation.

## Abstract

The anisotropic changes in the electronic structure of a metal-to-insulator transition (MIT) material, RuAs, with two-step phase transition are reported by using polarized optical conductivity [$\sigma(\omega)$] spectra, angle-integrated photoelectron (PE) spectra, and band calculations based on local density approximation (LDA). Both the PE and $\sigma(\omega)$ spectra not only in the high-temperature (HT) phase but also in the low-temperature (LT) phase as well as the energy gap formation owing to the MIT were almost consistent with those derived from the LDA band calculations, so the fundamental electronic structure in the HT and LT phases can be explained without electron correlations. However, the electronic structure in the middle phase between the HT and LT phases has not been clarified. The polarized $\sigma(\omega)$ spectra revealed not only the anisotropic energy gap formation but also the anisotropic gap-opening temperature, i.e., the energy gap along the $c$ axis in the HT phase starts to open near the higher transition temperature, but that along the $b$ axis opens below the lower transition temperature. The finding suggests that the two-step MIT originates from the anisotropic energy gap formation.