Quantum effects in the teraherz dynamics of supercritical ${}^4\mathrm{He}$ have been studied as a function of both density $\rho$ and temperature T; they have been characterized through their effects on the second and third spectral moments of the dynamic structure factor $S(Q,\omega ),$ 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.

• Received 3 October 2000

DOI:https://doi.org/10.1103/PhysRevE.64.021203