Graphene physics and insulator-metal transition in compressed hydrogen

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.