The breakdown of superfluidity in liquid 4He

A precise experimental determination of the Landau critical velocity upsilonsub(L) for roton creation in HeII is reported. The technique used was based on measurements of the drift velocity, upsilon-bar, of negative ions through isotopically pure liquid 4He at ca. 80 m K, under the influence of weak electric fields, E, for pressures, P, within the range 13 <= P <= 25 bar. It relied on the use of the equation (upsilon-bar-upsilonsub(L)) varies as Esup(1/3), which is believed to correspond to the creation of rotons occurring predominantly in pairs and which fitted the experimental data to very high precision for E > 500 V m-1. At lower values of E, however, small deviations from this equation were observed. These are tentatively attributed, not to the predicted onset of single-roton emission, but to a novel form of ion-vortex scattering. The values of upsilonsub(L)(P) deduced from the measurements of upsilon-bar(E) at various pressures for E > 500 V m-1 agree to within 1.5% with theoretical predictions based on Landau's excitation model of HeII, incorporating accepted numerical values of the roton parameters. The observed pressure dependence of upsilonsub(L)(P) is significantly stronger than that predicted; however, a discrepancy that appears to point towards the decreasing accuracy with which the roton parameters are known at high pressures. The modulus of the matrix element |Vsub(k0,k0)| characterizing roton-pair emission has also been deduced and is found to decrease rapidly with falling pressure. A linear extrapolation of the data suggests that |Vsub(k0,k0)| falls to zero at P approx. 3 bar (1 bar = 105 Pa). (author)