Quantum crystals are characterized by zero-point-energy motions large enough to explore the details of the interaction potential beyond the harmonic approximation even at zero temperature. By making recourse to ultrahigh-resolution neutron spectroscopy, we have probed simultaneously three observables sensitive to anharmonic effects in solid para-H 2 over a wide temperature range, namely, Bragg scattering, molecular root-mean-square displacements, and the spectral linewidth of the J=0→ 1 rotational transition which, on average, amounts to a half-width at half-maximum of 1.8±0.2 οeV. These three quantities show no measurable variation with temperature, thus signaling the existence of a fully expanded crystal across the entire solid phase. Our results provide unambiguous experimental evidence for the intrinsic quantum character of this fundamental molecular solid. © 2012 American Physical Society.

Work supported in part by Grant No. MAT2007-65711- C04-01 from the Spanish Ministerio de Ciencia e Innovación. FFA gratefully acknowledges the UK Science & Technology Facilities Council for financial support

Peer Reviewed