Quantum effects in the dynamics of He probed by inelastic x-ray scattering- art. no. 021203

Quantum effects in the teraherz dynamics of supercritical He-4 have been st udied as a function of both density rho and temperature T; they have been c haracterized through their effects on the second and third spectral moments of the dynamic structure factor S(Q,omega), measured by the inelastic x-ra y scattering (IXS) technique, The IXS spectra were collected in the low-Q r egion 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 measure d spectral moments clearly show a departure from their high-T classical exp ected values. We observe, moreover, that the amplitude of quantum deviation s increases slightly with increasing density. This experimental method allo ws us to extract, even in a region where the dynamics still maintains a col lective character, such typical single particle properties as the mean atom ic kinetic energy.