Optical spectroscopy and the nature of the insulating state of rare-earth nickelates

TL;DR

This study investigates the insulating state of rare-earth nickelates NdNiO3 and SmNiO3 using optical spectroscopy and transport measurements, revealing a two-peak spectral feature linked to bond-disproportionation and Mott physics.

Contribution

It combines experimental spectroscopy with theoretical calculations to elucidate the electronic structure and origin of the insulating phase in rare-earth nickelates.

Findings

Identification of a two-peak structure in optical spectra during metal-insulator transition

Confirmation of spectral features through dynamical mean-field theory

Insight into the role of bond-disproportionation and Mott physics in insulating state

Abstract

Using a combination of spectroscopic ellipsometry and DC transport measurements, we determine the temperature dependence of the optical conductivity of NdNiO$_3$ and SmNiO$_{3}$ films. The optical spectra show the appearance of a characteristic two-peak structure in the near-infrared when the material passes from the metal to the insulator phase. Dynamical mean-field theory calculations confirm this two-peak structure, and allow to identify these spectral changes and the associated changes in the electronic structure. We demonstrate that the insulating phase in these compounds and the associated characteristic two-peak structure are due to the combined effect of bond-disproportionation and Mott physics associated with half of the disproportionated sites. We also provide insights into the structure of excited states above the gap.