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Physical Review Letters

Motion of Ions Trapped on Vortices in He II

Richard M. Ostermeier, William I. Glaberson

Published Jul 28, 1975• 47 citations
Publisher

Abstract

We have measured the drift velocity of ions trapped on vortex lines at temperatures down to 0.35 K. We observe several effects which cannot easily be explained in terms of existing theories, including the broadening of an ion pulse, a limiting velocity in high electric fields, and a zero-field mobility which increases, at lower temperatures, much more rapidly than expected. An attempt is made to explain qualitatively some of these effects.

DOI
10.1103/physrevlett.35.241

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Physical Review A•Jul 1, 1972•67 citations
We have measured the drift velocity of ions trapped on vortex lines at temperatures down to 0.35 K. We observe several effects which cannot easily be explained in terms of existing theories, including the broadening of an ion pulse, a limiting velocity in high electric fields, and a zero-field mobility which increases, at lower temperatures, much more rapidly than expected. An attempt is made to explain qualitatively some of these effects.
The mobility of negative ions trapped on linear vortices in rotating superfluid helium has been measured by a time-of-flight technique in the temperature range $0.8\ifmmode^\circ\else\textdegree\fi{}\mathrm{K}<T<1.6\ifmmode^\circ\else\textdegree\fi{}\mathrm{K}$. Measurements could not be extended below 0.8\ifmmode^\circ\else\textdegree\fi{}K because, contrary to the present theory, the ion escape rate became too large. No evidence of direct trapping of positive ions by vortex lines was observed down to 0.4\ifmmode^\circ\else\textdegree\fi{}K. The trapped-ion mobility is interpreted in terms of scattering of the ion by thermal distributions of quantized vortex waves and rotons localized about the vortex axis.
Abstract At higher electric fields, the drifting ions are not in thermal equilibrium with the gas of the elementary excitations of the superfluid, and the mobility depends on the field. This chapter presents the experimental results of the ion mobility in the superfluid at higher fields and larger temperatures. The field dependence of the mobility is explained in terms of roton scattering in presence of an enhanced roton density around the charge. Emphasis is given on the discovery of the giant mobility discontinuity at which the excess energy is dissipated by ions creating quantized vortex rings. Ions are captured by them and drift together as a new, single unit called a charged vortex ring. The puzzling issue of the drift velocity discontinuities is also addressed.
Negative-ion trapping has been investigated in rotating liquid helium at temperatures above 1.2\ifmmode^\circ\else\textdegree\fi{}K. We report measurements of the ion-capture width versus temperature and electric field (from 5 to 100 V/cm). The theory of the effect is reviewed and discussed and is shown to be in excellent agreement with experiment. Owing mainly to the insensitivity, at low ratios of electric field to temperature, of the capture width to the properties of the ion-vortex potential well, we are able to deduce only that the well must be larger than the ionic mean free path.
Abstract This chapter describes the phenomena observed when the drift field is large enough for ions to escape capture by vortex rings. The electric field dependence of the ion drift velocity at high fields can be interpreted in terms of a sequence of trapping and detrapping events. At very high field, ions can drift without being captured by vortex rings, and eventually they may reach velocities in excess of the Landau critical velocity for roton emission. The breaking of the roton barrier is described in great detail because of its importance. It is the first time that a macroscopic critical velocity has been reached in the superfluid by a moving object. The microscopic size of the ions has allowed this impressive goal to be reached.
Measurements of the velocity of charge carriers in several dilute solutions of 3He^{3}\mathrm{He}3He in 4He^{4}\mathrm{He}4He in the temperature range 0.017-1.0 K are reported. The low-field negative-ion mobility shows a ${T}^{\ensuremath{-}\frac{1}{2}}$ dependence below 0.08 K characteristic of hard-sphere scattering. A maximum is observed in the mobility in the vicinity of 0.4 K. This can be interpreted in terms of a 3He^{3}\mathrm{He}3He condensation on the bubble surface. The positive-ion mobility is nearly temperature independent below 0.7 K. This has recently been interpreted in terms of a momentum-dependent polarization scattering. The field dependence of the positive-carrier velocity shows a discontinuity in the vortex-ring regime. The discontinuity can be shown to result from a size-dependent drag coefficient and a maximum is observed for very small rings (\ensuremath{\sim}16 \AA{}).