T. Ellis

The breakdown of superfluidity in liquid 4He V. Measurement of the Landau critical velocity for roton creation

We report a precise experimental determination of the Landau critical velocity vL for roton creation in Hen. The technique used was based on measurements of the drift velocity, v, 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 (v— vL) oc E1/3, 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 vL(P) deduced from the measurements of v(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 vL(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 |Vk0,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 |Vk0,k0| falls to zero at P « 3 bar (1 bar = 105 Pa).

Measurement of the Landau velocity in He II

We report the preliminary results of an experiment to measure the Landau critical velocity in He II. Our measurements, for pressures P in the range 19 < P < 25 bar, agree to better than 1% with values of vL calculated from accepted values of the Landau parameters, but lie systematically above them.

The breakdown of superfluidity in liquid 4He. II. An investigation of excitation emission from negative ions travelling at extreme supercritical velocities

We have measured the drift velocity, v, of negative ions in He II at a temperature of 0.34 K and a pressure of 25 bar¶ for electric fields, E, in the range 5 x 103 < E < 6 x 106 V m-1. For v <70 m s-1, the results are in good agreement with the predictions of the roton pair-emission theory of Bowley & Sheard. Even better agreement is obtained by extending the theory to take explicit account of departures from parabolicity of the real dispersion curve at large momenta, variations in the pole strength of high momentum excitations, and the slight increase in the average momentum of excitations emitted at large values of v. For v> 70 m s-1, however, experiment and theory are in clear disagreement: by v = 80 m s-1, the measured drag on the ion has become about 100 % larger than the theoretical prediction. We tentatively attribute this discrepancy to the onset at v« 70 m s-1 of a new and, so far, unidentified dissipation mechanism.

Pressure dependence of the negative ion effective mass in He II

The pressure dependence of the effective mass m* of the negative ion in He II has been investigated for the first time. On this assumption of a free-particle-like ionic dispersion relation, the values of m* deduced from the measurements range from 108 m4 at 11 bar to 77 m4 at 25 bar, where m4 is the 4He atomic mass. There are indications in the raw data, however, that there may be a small quartic term in the dispersion relation. If the data are re-analysed on the assumption that such an extra term does exist, the derived values of m* are reduced by about 8%, which then brings them into excellent agreement with the predictions of the bubble model of the negative ion.

Search for single-roton emission from negative ions in HeII1

Abstract We describe an experiment to search for evidence of single-roton emission from negative ions in HeII. Preliminary results are presented, enabling an upper bound of 3 × 10 −3 to be placed on the ratio of the single/pair emission rate constants. We draw attention to the urgent need for more accurate measurements of the Landau parameters at low temperatures close to the melting pressure.

Free particle dynamics of negative ions in the mechanical vacuum of He II

We have employed a fast field reversal technique to demonstrate the existence of inertial effects in the motion of negative ions through isotopically pure He II at 70 mK.