Graphene physics and insulator-metal transition in compressed hydrogen

TL;DR

This paper explores how layered structures in compressed hydrogen, resembling distorted graphene sheets, undergo insulator-metal transitions driven by electronic instabilities and band crossing phenomena.

Contribution

It introduces a model-based understanding of the electronic structure and metallization mechanisms in compressed hydrogen's layered phases.

Findings

Layered hydrogen structures can be modeled as distorted graphene sheets.

Electronic instabilities lead to structural distortions and band gap openings.

Metallization occurs via band crossing and formation of $2p_z$ $c6$ bonding states.

Abstract

Compressed hydrogen passes through a series of layered structures in which the layers can be viewed as distorted graphene sheets. The electronic structures of these layered structures can be understood by studying simple model systems- an ideal single hydrogen graphene sheet and three-dimensional model lattices consisting of such sheets. The energetically stable structures result from structural distortions of model graphene-based systems due to electronic instabilities towards Peierls or other distortions associated with the opening of a band gap. Two factors play crucial roles in the metallization of compressed hydrogen: (i) crossing of conduction and valence bands in hexagonal or graphene-like layers due to topology and (ii) formation of bonding states with $2p_z$ $\pi$ character.