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Physical Review Letters

Evidence for Roton Pair Creation in Superfluid He4

D. R. Allum, Roger M. Bowley, P. V. E. McClintock

Published May 31, 1976• 75 citations
Publisher

Abstract

We have measured the drift velocity $\overline{v}$ of negative ions through superfluid $^{4}\mathrm{He}$ for temperatures $T<0.45$ 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 $\overline{v}(E)$, 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.

DOI
10.1103/physrevlett.36.1313

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•May 31, 2007•0 citations
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×\else\texttimes\fi10420<E<2.6\ifmmode\times\else\texttimes\fi{}{10}^{4}20<E<2.6\ifmmode×\else\texttimes\fi104 V cm\ensuremath−1{\mathrm{cm}}^{\ensuremath{-}1}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.
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 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.
Liquid helium and spin-1/2 cold-atom Fermi gases both exhibit in their superfluid phase two distinct types of excitations, gapless phonons and gapped rotons or fermionic pair-breaking excitations. In the long wavelength limit, revising and extending the theory of Landau and Khalatnikov initially developed for helium [Zh. Exp. Teor. Fiz. 19, 637 (1949)], we obtain universal expressions for three- and four-body couplings among these two types of excitations. We calculate the corresponding phonon damping rates at low temperature and compare them to those of a pure phononic origin in high-pressure liquid helium and in strongly interacting Fermi gases, paving the way to experimental observations.
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.