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Physical Review A

Ion Motion in Dilute He3 - He4 Solutions at Ultralow Temperatures

Moyses Kuchnir, John B. Ketterson, Pat R. Roach

Published Jul 1, 1972• 67 citations
Publisher

Abstract

Measurements of the velocity of charge carriers in several dilute solutions of $^{3}\mathrm{He}$ in $^{4}\mathrm{He}$ 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 $^{3}\mathrm{He}$ 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{}).

DOI
10.1103/physreva.6.341

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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.

•May 31, 2007•0 citations
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−12{T}^{\ensuremath{-}\frac{1}{2}}T\ensuremath−21​ 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{}).
Measurements of the positive-ion velocity as a function of electric field in dilute solutions of 3He^{3}\mathrm{He}3He in 4He^{4}\mathrm{He}4He at 18 mK show a discontinuity which suggests the existence of two kinds of vortex rings in addition to the bare ion. The assignment of circulation quantum numbers is discussed.
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.
We describe an experiment which indicates that the addition of He3{\mathrm{He}}^{3}He3 impurities to superfluid helium has a pronounced effect on the velocity at which negative ions produce vorticity (vortex rings). The He3{\mathrm{He}}^{3}He3 impurities have no effect, however, on the creation of vorticity by positive ions.
We calculate the roton-limited mobility of ions in superfluid He4{\mathrm{He}}^{4}He4 for temperatures below \ensuremath{\sim} 1.7\ifmmode^\circ\else\textdegree\fi{}K. Taking, as a first approximation, a constant ion-roton transition-matrix element, which we treat as an adjustable parameter, we find good agreement with experiment for both positive and negative ions. This model calculation indicates that the most mobile of the recently discovered "exotic" negative carriers is a very light ion.
The lifetimes of both positive and negative ions trapped on quantized vortex lines in rotating 3He^{3}\mathrm{He}3He-4He^{4}\mathrm{He}4He mixtures have been measured at temperatures down to 0.1 K. Evidence is found that the 3He^{3}\mathrm{He}3He condenses at the vortex core, and possibly at the surface of the electron bubble.
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.