Quantum phase transition in solid hydrogen at high pressure

Abstract

Extensive experimental and theoretical studies have been devoted to determining the high-pressure phase diagram of hydrogen. We present evidence of a phase at a higher pressure than phase III and below the pressure of the recently observed phase of metallic hydrogen (495 GPa). This phase was determined from infrared (IR) spectroscopy of hydrogen samples at static pressures above 360 \ifmmode\pm\else\textpm\fi{} 15 GPa in a diamond anvil cell, and has been observed in three separate experiments. Whereas earlier studies found new high-pressure phases that only occurred at elevated temperatures, this phase transition occurs at the lowest temperatures investigated, \ensuremath{\sim}5 K, and the steep phase line indicates that it is a quantum phase transition. This phase is characterized by two distinct IR absorption bands (2950 and $3335\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ at 365 GPa). Above the transition pressure we observe strong darkening of the sample in the visible spectrum as pressure is increased. Observations are compatible with the cmca-12 crystal structure.