Negative ion motion in normal and superfluid 3He

We report the first measurements of negative ion motion in the superfluid phases of ³He and in the normal phase below 17 mK. Refrigeration was achieved with nuclear demagnetization of copper and we used a pulsed NMR platinum powder thermometer immersed in the liquid. In the A phase the longitudinal resonance frequency provided an additional high-resolution thermometer. In the normal phase we observed a strictly temperature-independent mobility. In the superfluid phases we found two velocity regimes. For small applied electric fields the velocity is a linear function of the field and the corresponding mobility increases monotonically toward lower temperatures. At high electric fields the velocity is a nonlinear function of the field as a result of the pair-breaking effect of the moving ion. Available theoretical calculations are only in partial agreement with our results.This work was supported financially by the Academy of Finland.Guggenheim Fellow on leave from Cornell University, Ithaca, New York.On leave from Regensburg University, Regensburg, West Germany, supported by Deutsche Forschungsgemeinschaft and Finnish Ministry of Education.