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BARE-ION TRANSPORT AT HIGH FIELDS

A. F. Borghesani

Published May 31, 2007• 0 citations
Publisher

Abstract

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.

DOI
10.1093/acprof:oso/9780199213603.003.0010

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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.
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.
We have measured the drift velocity v‾\overline{v}v of negative ions through superfluid 4He^{4}\mathrm{He}4He for temperatures T<0.45T<0.45T<0.45 K, under a pressure of 25 bar, for electric fields $20<E<2.6\ifmmode\times\else\texttimes\fi{}{10}^{4}$ V ${\mathrm{cm}}^{\ensuremath{-}1}$. From the observed form of v‾(E)\overline{v}(E)v(E), we deduce that roton emission from an ion traveling faster than the Landau critical velocity is governed by hitherto unrecognized selection rules whereby the rotons can be created only in pairs.
Abstract This chapter examines the wealth of experiments on the nucleation of vortex rings in the vicinity of the giant discontinuity. The nucleation in pure superfluid and in dilute isotopic mixtures has been investigated and three main models have been theoretically developed to interpret the experimental data: the peeling, the girdling, and the fluctuation models. The effects of isotopic impurities, high fields, and high pressures have been investigated and the results are described. The more recent experiments have elucidated the mechanisms of vortex nucleation, including the roton emission at velocities exceeding the Landau critical velocity and the possibility of macroscopic quantum tunnelling over a barrier. The enormous influence of isotopic impurities is also described.
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.