The mobility of positive ions has been measured in the normal and superfluid 3 , ,
phases of He at several pressures. Below 100 mK the normal phase mobthty increases logarithmically with decreasing temperature down to the superfluid transition temperature T~; it shows an anomalous fump near 100 inK. At low temperatures the drift velocity is nonlinear for electric fields exceeding 30 V/cm. In the superfluid the mobility, normalized to its value at To, is much less than for negative ions. We have also observed the anisotropic mobility in the A phase and the Landau critical velocity for pair-breaking in both superfluid phases.
In our previous measurements ~2 of negative ion mobility in liquid 3He, no trace of ionic recoil effects, which should reduce the scattering of quasiparticles at low temperatures, could be distinguished in the normal phase. Positive ions are expected to be lighter than negative ions, offering further insight into the effects of energy exchange between 3He quasiparticles and the ion. For this reason we have extended the measurements to positive ions both in the normal and the superfluid phases. Indeed we have found that the positive ion mobility increases with decreasing temperature in the normal phase. In the superfluid phases we observed qualitatively similar behavior as in the case of negative ions. The enhancement of the mobility, normalized to the value at the superfluid transition temperature, however, was clearly less pronounced for positive ions. During the course of our work, Roach et al. 3"4 reported similar experiments with negative and positive ions. We will compare their results with ours. Our normal phase data at low temperature, in particular, disagree with theirs and we will discuss the reasons for this discrepancy. Most recently Alexander et al. 5 studied positive ions in the normal phase and observed novel features