Quantum effects in the dynamics of He probed by inelastic x-ray scattering

Quantum effects in the teraherz dynamics of supercritical ⁴He have been studied as a function of both density ρ and temperature T; they have been characterized through their effects on the second and third spectral moments of the dynamic structure factor S(Q,ω), measured by the inelastic x-ray scattering (IXS) technique. The IXS spectra were collected in the low-Q region below and around the position of the first diffraction peak Q_m, i.e., in a range relatively unusual in this kind of investigation. The measured spectral moments clearly show a departure from their high-T classical expected values. We observe, moreover, that the amplitude of quantum deviations increases slightly with increasing density. This experimental method allows us to extract, even in a region where the dynamics still maintains a collective character, such typical single particle properties as the mean atomic kinetic energy.