Quantum effects in the single-particle kinetic energy of high-density fluid He-4

Deep inelastic neutron-scattering measurements have been performed in dense fluid He-4 along three isochores at number densities 33, 38, and 45 nm(-3) and temperatures ranging from 4.35 to 56 K. Data have been analyzed in the plane-wave impulse-approximation framework to obtain the root-mean-square values of the single-particle momentum along a given direction p(x). The density and temperature behavior of the derived mean kinetic energies is discussed in comparison with a simple harmonic model for the fluid and with quantum simulations performed using a path-integral Monte Carlo code. The comparison with the harmonic models sheds Light on the density dependence of the zero-point kinetic energy of the particles and points out the inadequacy of a single-frequency Einstein-oscillator model in describing its temperature dependence. The quantum simulation results are in substantial agreement with the experimental data although systematic: deviations are found as the density is increased.