Road to Hot Hydride Superconductors*

The realization of superconductivity in dense hydrides in the vicinity of room temperature dates back to predictions of very high T_c in atomic metallic hydrogen and subsequently in doped metallic hydrogen alloys and compounds. Density-functional based structure-search methods calculations combined with BCS-type models predicted a series of dense hydride high T_c superconductors (e.g., CaH₆, H₃S) at megabar pressures (>100 GPa), followed by the still higher T_c superhydrides (e.g., LaH_x, YH_x, with x > 6). Using a variety of x-ray diffraction and transport measurements, we confirmed our predictions of the stability of LaH₁₀ and its T_c to 260 K near 200 GPa as well as superconductivity in other La-H phases, results that were subsequently confirmed independently. Additional studies have been carried out on high T_c hydrides of Se, Ca, Se-S, and other ternary systems. The results are consistent with conventional superconductivity, though the mechanism is likely affected by strong quantum effects in these systems. These efforts highlight the novel physics of metallic hydrogen-like materials, the success of ‘materials by design’ using high pressures, and the possibility of superconductivity well above room temperature as recently predicted.