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

We report novel results from inelastic neutron scattering measurements at very high momentum transfer, q less than or equal to 122 Angstrom-1, performed in supercritical 4He, 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 experimental data have been analysed within the framework of the plane-wave Impulse Approximation and the temperature dependence of the single-particle mean kinetic energy has been derived. It is found that this quantity deviates from the classical behaviour in the whole temperature range explored, showing the quantum nature of this system even for these very low-density isochores, the lowest ever experimentally explored. For the higher density, the temperature evolution of the single-particle mean kinetic energy has also been 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, at these densities, the temperature dependence of the mean kinetic energy is consistent with an anharmonic behaviour of 4He induced by the hard-core component of the interatomic potential.