Theory for Metal Hydrides with Switchable Optical Properties

TL;DR

This paper develops a many-body theoretical model for metal hydrides, especially lanthanum hydride, explaining their switchable optical properties and metal-insulator transition through strong electron correlation and vacancy states.

Contribution

It introduces a large-U Anderson lattice model to describe the electronic structure and optical behavior of metal hydrides, providing new insights into their phase transition mechanisms.

Findings

LaH₃ is an insulator with a substantial energy gap.

Vacancy states are highly localized due to strong bonding.

Metal-insulator transition occurs within vacancy states band.

Abstract

Recently it has been discovered that lanthanum, yttrium, and other metal hydride films show dramatic changes in the optical properties at the metal-insulator transition. Such changes on a high energy scale suggest the electronic structure is best described by a local model based on negatively charged hydrogen (H$^-$) ions. We develop a many-body theory for the strong correlation in a H$^-$ ion lattice. The metal hydride is described by a large $U$-limit of an Anderson lattice model. We use lanthanum hydride as a prototype of these compounds, and find LaH$_3$ is an insulator with a substantial gap consistent with experiments. It may be viewed either as a Kondo insulator or a band insulator due to strong electron correlation. A H vacancy state in LaH$_3$ is found to be highly localized due to the strong bonding between the electron orbitals of hydrogen and metal atoms. Unlike the impurity states in the usual semiconductors, there is only weak internal optical transitions within the vacancy. The metal-insulator transition takes place in a band of these vacancy states.