  A possibility of high, room-temperature superconductivity was predicted for
metallic hydrogen in the 1960s. However, metallization and superconductivity of
hydrogen are yet to be unambiguously demonstrated in the laboratory and may
require pressures as high as 5 million atmospheres. Rare earth based
"superhydrides" such as LaH10 can be considered a close approximation of
metallic hydrogen even though they form at moderately lower pressures. In
superhydrides the predominance of H-H metallic bonds and high superconducting
transition temperatures bear the hallmarks of metallic hydrogen. Still,
experimental studies revealing the key factors controlling their
superconductivity are scarce. Here, we report on the pressure and magnetic
field response of the superconducting order observed in LaH10. For LaH10 we
find a correlation between superconductivity and a structural instability,
strongly affecting the lattice vibrations responsible for the
superconductivity.
