Molecular mean-squared displacement in solid parahydrogen

The condensed phases of molecular hydrogen are systems of fundamental importance in quantum many-body physics. Zero-point motion makes a large contribution to the kinetic energy and mean-squared displacement <u²> of hydrogen molecules in the liquid and solid phases. At present, there is a significant disagreement between theory and experiment about the relative importance of thermal and quantum effects near the liquid-solid phase transition of parahydrogen. Path Integral Monte Carlo calculations predict that <u²> increases as the melting temperature is approached from below, implying a combination of thermal and quantum effects [Phys. Rev. B 95, 104518 (2017)]. However, an inelastic neutron scattering study of the rotational transitions in solid parahydrogen suggests that <u²> is independent of temperature, and is thus determined by quantum-mechanical zero-point motion alone [Phys. Rev B 86, 144524 (2012)]. In this presentation, we report new inelastic neutron scattering measurements of <u²> obtained using the ARCS spectrometer at Oak Ridge National Laboratory and the DCS spectrometer at the NIST Center for Neutron Research.