Semimetal states of crystalline molecular hydrogen at high pressures

Semimetal states of crystalline molecular hydrogen have been obtained at a temperature of 100 K in the pressure range from 410 to 626 GPa. To analyze the nature of conductivity, the band structure is calculated in the framework of the density functional theory using the Heyd-Scuseria-Ernzerhof hybrid exchange-correlation functional. One of the semimetal states occurs in a monoclinic structure with C2/c symmetry under compression to a pressure of 410 GPa, at which the gap between the valence and conduction bands is closed. Moreover, the valence band is partially unfilled, and the conduction band is partially filled, which is a characteristic feature of a semimetal. At a pressure of 302 GPa, crystalline molecular hydrogen with the C2/c structure remains an insulator. The pressure dependence of the electrical conductivity in the range of 300–500 GPa is determined. The second semimetal state is observed for a rhombic structure with Cmca-4 symmetry at a pressure of 626 GPa.