Quantum momentum distribution and kinetic energy in solid 4He -: art. no. 075301

We present measurements of neutron scattering from solid He-4 at high momentum transfer. The solid is held close to the melting line at molar volume 20.87 cm(3)/mol and temperature T=1.6 K. From the data, we determine the shape of the momentum distribution, n(k), of atoms in the solid and the leading final state contribution to the scattering. We show that n(k) in this highly anharmonic, quantum solid differs significantly from a Gaussian. The n(k) is more sharply peaked with larger occupation of low momentum states than in a Maxwell-Boltzmann distribution, as found in liquid He-4 and predicted qualitatively by path integral Monte Carlo calculations. The atomic kinetic energy is <K>=(24.25+/-0.30) K. If n(k) is assumed to be Gaussian, as is usually the practice, a <K> 10% smaller is obtained.