Graphene physics and insulator-metal transition in compressed hydrogen

As established recently both theoretically and experimentally, compressed hydrogen passes through a series of layered structures in which the layers can be viewed as distorted graphene sheets. These structures and their electronic properties can be understood by studying simple model systems-(i) a H$_6$ ring, (ii) an ideal single hydrogen graphene sheet and (iii) three-dimensional model lattices consisting of such sheets [1]. 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 bandgap. Two factors play crucial roles in the metallization of compressed hydrogen: (i) crossing of conduction and valence bands in hexagonal or grapheme-like layers due to topology and (ii) formation of bonding states with 2p${_z}$ $\pi$ character.\\[4pt] [1] I. I. Naumov, R. E. Cohen and R. J. Hemley Phys. Rev. B, {\bf 88}, 045125 (2013).