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Physical Review A

Roton Trapping by a Vortex Line

Ienari Iguchi

Published Sep 1, 1972• 33 citations
Publisher

Abstract

A model Hamiltonian is used to find the dispersion relation of trapped rotons under the influence of a vortex field by a variational technique. The lowest branch corresponds to the energy gap \ensuremath{\sim} 2 \ifmmode^\circ\else\textdegree\fi{}K, in good agreement with the result obtained by previous semiclassical theory. The trapped mode explains the recent mobility measurements of trapped ions well.

DOI
10.1103/physreva.6.1087

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Abstract Ions and electron bubbles are captured by the vortex lines which occur in a rotating superfluid sample. Once captured, however, the charges can move along the quantized vortex lines. On one hand, this degree of freedom allows the researcher to use ions to detect the appearance of single, quantized vortex lines. On the other hand, the study of the ion motion along the lines gives important pieces of information on the structure and dynamics of vortices. In fact, the ion motion along the lines is limited by scattering on 3He impurities captured by the vorticity field and by vortex waves, i.e., columnar oscillation of the vortex lines. Experiments and theory of vortex wave-, roton, and 3He impurity scattering on vortex lines are described.

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A model Hamiltonian is used to find the dispersion relation of trapped rotons under the influence of a vortex field by a variational technique. The lowest branch corresponds to the energy gap \ensuremath{\sim} 2 \ifmmode^\circ\else\textdegree\fi{}K, in good agreement with the result obtained by previous semiclassical theory. The trapped mode explains the recent mobility measurements of trapped ions well.
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 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.
We apply a recently developed effective string theory for vortex lines to the case of two-dimensional trapped superfluids. We do not assume a perturbative microscopic description for the superfluid, but only a gradient expansion for the long-distance hydrodynamical description and for the trapping potential. For any regular trapping potential, we compute the spatial dependence of the superfluid density and the orbital frequency and trajectory of an off-center vortex. Our results are fully relativistic and in the nonrelativistic limit reduce to known results based on the Gross-Pitaevskii model. In our formalism, the leading effect in the nonrelativistic limit arises from two simple Feynman diagrams in which the vortex interacts with the trapping potential through the exchange of hydrodynamical modes.
Abstract Ions and electron bubbles are captured by the vortex lines which occur in a rotating superfluid sample. Once captured, however, the charges can move along the quantized vortex lines. On one hand, this degree of freedom allows the researcher to use ions to detect the appearance of single, quantized vortex lines. On the other hand, the study of the ion motion along the lines gives important pieces of information on the structure and dynamics of vortices. In fact, the ion motion along the lines is limited by scattering on 3He impurities captured by the vorticity field and by vortex waves, i.e., columnar oscillation of the vortex lines. Experiments and theory of vortex wave-, roton, and 3He impurity scattering on vortex lines are described.
We have studied negative-ion absorption by rotating He II from 0.98\ifmmode^\circ\else\textdegree\fi{}K to 1.7\ifmmode^\circ\else\textdegree\fi{}K. Measurements of vortex-line capture cross sections are reported as a function of temperature (0.98\ifmmode^\circ\else\textdegree\fi{}K to 1.7\ifmmode^\circ\else\textdegree\fi{}K at three electric fields (16.3, 49.0, and 97.9 V/cm) and as a function of electric field (10 to 200 V/cm) at two temperatures. Great care was taken to insure that the reported cross-section values were not affected by space-charge effects. The results are found in good agreement with the ion-capture theories developed by Donnelly and Roberts. The decrease in cross section above 1.6\ifmmode^\circ\else\textdegree\fi{}K is in agreement with the escape calculation of Parks and Donnely and supports their value of 15.96 \AA{} for the negative-ion radius.