Single-particle mean kinetic energy in low-density supercritical He-4

We report novel results from inelastic neutron scattering measurements at v ery high momentum transfer, q less than or equal to 122 Angstrom(-1), perfo rmed in supercritical He-4, along two very low-density isochores (rho = 10. 35 nm(-3) and rho = 13.8 nm(-3)) in the temperature range 5-30 K. The exper imental data have been analysed within the framework of the plane-wave Impu lse Approximation and the temperature dependence of the single-particle mea n kinetic energy has been derived. It is found that this quantity deviates from the classical behaviour in the whole temperature range explored, showi ng the quantum nature of this system even for these very low-density isocho res, the lowest ever experimentally explored. For the higher density, the t emperature evolution of the single-particle mean kinetic energy has also be en derived using a path-integral Monte Carlo code. These quantum simulation results, as well as those already available for the low-density isochore, are in good agreement with the experimental data. It is pointed out that, a t these densities, the temperature dependence of the mean kinetic energy is consistent with an anharmonic behaviour of He-4 induced by the hard-core c omponent of the interatomic potential.