Helium-bearing superconductor at high pressure

Abstract

Helium (He) is the most inert noble gas at ambient conditions. It was predicted to adopt a hexagonal close packed structure ($P{6}_{3}/mmc$) and remains in the insulating phase up to 32 TPa. In contrast, lithium (Li) is one of the most reactive metals at zero pressure, while its cubic high-pressure phase ($Fd\overline{3}m$) is a weak metallic electride above 475 GPa. Strikingly, computations predict a stable compound of ${\mathrm{Li}}_{5}{\mathrm{He}}_{2}$ ($R\overline{3}m$) by mixing $Fd\overline{3}m$ Li with $P{6}_{3}/mmc$ He above 700 GPa from ab initio evolutionary searches. The presence of helium promotes the lattice transformation from $Fd\overline{3}m$ Li to $Pm\overline{3}m$ Li, and turns the three-dimensional distributed interstitial electrons into the mixture of zero- and two-dimensional anionic electrons. This significantly increases the degree of metallization at the Fermi level; consequently, the coupling of conductive anionic electrons with the Li-dominated vibrations is the key factor to the formation of superconducting electride ${\mathrm{Li}}_{5}{\mathrm{He}}_{2}$ with a transition temperature up to 26 K, dynamically stable to pressures down to 210 GPa.