Observation of a new strongly correlated phase of H2

Hard Condensed Matter Seminar: Dr. Raina Olsen, Oak Ridge National Laboratory

We report direct evidence of a new phase of molecular hydrogen (H₂) observed in a system of H₂ adsorbed in a graphitic nano-porous carbon at temperatures (74-92 K) and pressures (>76 bar) well above the critical point of bulk H₂.  The system is studied with deep inelastic neutron scattering (DINS), where ‘deep’ refers to the high energy of the incident neutrons which penetrate deeply into the system to study its individual atoms within their local environment.  Normally scattering becomes fully incoherent, reflecting only correlations of a single particle with itself at a later time, at momentum transfers Q>10-12 Å^-1, where 1/Q is smaller than the length scale of the thermal fluctuations of the atoms.  But in this new phase, we observe novel spectral features that remain coherent, reflecting inter-atomic correlations, up to the highest momentum transfers measured, 35 Å^-1.  This corresponds to length scales several times smaller than the H-H bond, indicating that the system has transitioned to a strongly correlated state.  Hydrogen adsorption measurements also show evidence of an exothermic phase transition with H₂ denser in the novel phase.  We present the results of several experiments on this system, then propose a model of H₂-H₂ nuclear spin correlations mediated through a quantum exchange interaction to explain the measured properties.  Future work needed to further probe the system and prove the existence of nuclear spin correlations is discussed.