title: Topological Surface States in Dense Solid Hydrogen.
abstract: Metallization of dense hydrogen and associated possible high-temperature superconductivity represents one of the key problems of physics. Recent theoretical studies indicate that before becoming a good metal, compressed solid hydrogen passes through a semimetallic stage. We show that such semimetallic phases predicted to be the most stable at multimegabar (∼300  GPa) pressures are not conventional semimetals: they exhibit topological metallic surface states inside the bulk "direct" gap in the two-dimensional surface Brillouin zone; that is, metallic surfaces may appear even when the bulk of the material remains insulating. Examples include hydrogen in the Cmca-12 and Cmca-4 structures; Pbcn hydrogen also has metallic surface states but they are of a nontopological nature. The results provide predictions for future measurements, including probes of possible surface superconductivity in dense hydrogen.
authors.affiliation: Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C. 20015, USA.
authors.forename: Ivan I
authors.initials: II
authors.lastname: Naumov
authors.affiliation: Department of Civil and Environmental Engineering, The George Washington University, Washington, D.C. 20052, USA.|Lawrence Livermore National Laboratory, Livermore, California 94550, USA.
authors.forename: Russell J
authors.initials: RJ
authors.lastname: Hemley