Neutron scattering studies of the dynamic structure of liquid 4He

Abstract

Several aspects of the dynamic structure of liquid 4He have been studied by means of neutron inelastic scattering at selected temperatures, pressures, and values of the wave vector transfer Q. In particular, the Van Hove scattering function S(Q,ω) has been determined for Q = 3.0 nm−1 at 1.2 and 2.3 K at saturated vapor pressure (SVP), for Q = 8.0 nm−1 at 1.2 K and SVP, and for Q = 11.3 nm−1 (near the first maximum of the dispersion relation) and Q = 20.5 nm−1 (near the roton minimum) at 1.2 K and pressures of 0.07 and 2.4 MPa (just below the melting pressure). S(Q, ω) changes markedly with pressure; most strikingly, the intensity of the sharp 'one-phonon' peak at Q = 11.3 nm−1 decreases by about a factor of two between 0.07 and 2.4 MPa. For Q = 3.0 nm−1 and T = 1.2 K, the 'multiphonon' peak is sharp and symmetrical and centred at a frequency significantly greater than twice the roton-minimum frequency. For the higher Q values, the 'multiphonon' component exhibits a complex shape with a long high-frequency tail. The results appear to be describable in terms of one-phonon and multiphonon scattering with no indication that it is necessary to invoke the existence of two-roton bound states. They do, however, indicate that there is a strong interference between one-phonon and multiphonon scattering processes which in turn indicates that liquid helium is very 'anharmonic'.