False metals, real insulators, and degenerate gapped metals

TL;DR

This paper investigates compounds predicted as metals but are insulators, revealing that proper symmetry considerations in electronic structure theory can accurately describe their insulating nature, and introduces the concept of degenerate gapped metals.

Contribution

It clarifies the role of symmetry breaking in correctly predicting insulating states and introduces the concept of degenerate gapped metals with internal band gaps.

Findings

Symmetry restrictions can lead to incorrect metallic predictions.

Proper symmetry treatment reveals insulating phases in certain compounds.

Degenerate gapped metals have an internal band gap despite Fermi level positioning.

Abstract

This paper deals with a significant family of compounds predicted by simplistic electronic structure theory to be metals but are, in fact, insulators. This false metallic state has been traditionally attributed in the literature to reflect the absence of proper treatment of electron-electron correlation ("Mott insulators") whereas, in fact, even mean-field like density functional theory describes the insulating phase correctly if the restrictions posed on the simplistic theory are avoided. Such unwarranted restrictions included different forms of disallowing symmetry breaking described in this article. As science and technology of conductors have transitioned from studying simple elemental metals such as Al or Cu to compound conductors such as binary or ternary oxides and pnictides, a special class of degenerate but gapped metals has been noticed. Their presumed electronic configurations show the Fermi level inside the conduction band or valence band, yet there is an "internal band gap" between the principal band edges. The significance of this electronic configuration is that it might be unstable towards the formation of states inside the internal band gap when the formation of such states costs less energy than the energy gained by transferring carriers from the conduction band to these lower energy acceptor states, changing the original (false) metal to an insulator.