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Physical Review Letters

Focusing of Negative Ions by Vortices in Rotating He3−A

Juha Simola, Kaj K. Nummila, A. Hirai, J. S. Korhonen, W. Schoepe, L. Skrbek

Published Oct 13, 1986• 93 citations

Abstract

Experiments with negative ions in rotating superfluid $^{3}\mathrm{He}\ensuremath{-}A$ show strong retardation and deformation of ion pulses drifting parallel to the angular velocity $\ensuremath{\Omega}$. This is caused by the interaction between the ions and the $\mathbf{l}$ texture in the soft cores of $A$-phase vortices. The interaction is mediated by the anisotropic ion mobility: $\mathbf{v}={\ensuremath{\mu}}_{\ensuremath{\perp}}\mathbf{E}\ensuremath{-}\ensuremath{\Delta}\ensuremath{\mu}(\mathbf{E}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{l})\mathbf{l}$. The ($\ensuremath{\Omega},T$) dependence of the retardation is explained by a model that assumes focusing of ions into the vortex cores, along which the mobility ${\ensuremath{\mu}}_{c}$ is lower than that in the bulk liquid, ${\ensuremath{\mu}}_{\ensuremath{\perp}}$. We find ${\ensuremath{\mu}}_{c}={\ensuremath{\mu}}_{\ensuremath{\perp}}\ensuremath{-}0.6\ensuremath{\Delta}\ensuremath{\mu}$.

DOI: 10.1103/physrevlett.57.1923

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Experiments with negative ions in rotating superfluid

^{3}\mathrm{He}\ensuremath{-}A

show strong retardation and deformation of ion pulses drifting parallel to the angular velocity

\ensuremath{\Omega}

. This is caused by the interaction between the ions and the

\mathbf{l}

texture in the soft cores of

A

-phase vortices. The interaction is mediated by the anisotropic ion mobility:

\mathbf{v}={\ensuremath{\mu}}_{\ensuremath{\perp}}\mathbf{E}\ensuremath{-}\ensuremath{\Delta}\ensuremath{\mu}(\mathbf{E}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{l})\mathbf{l}

. The (

\ensuremath{\Omega},T

) dependence of the retardation is explained by a model that assumes focusing of ions into the vortex cores, along which the mobility

{\ensuremath{\mu}}_{c}

is lower than that in the bulk liquid,

{\ensuremath{\mu}}_{\ensuremath{\perp}}

. We find

{\ensuremath{\mu}}_{c}={\ensuremath{\mu}}_{\ensuremath{\perp}}\ensuremath{-}0.6\ensuremath{\Delta}\ensuremath{\mu}

.

The interaction between negative ions and 3 He superfluids was studied by using a time-of-flight spectrometer having a 10−3 relative resolution. The ions guided through a sample of rotating3He-A were found to be focused into the cores of the vortices present in this rotating superfluid. Two different types, with different core structures and presumably different numbers of circulation quanta associated with them, were found. The ion mobility along the vortex cores, μ c , observed in the absence of external magnetic field, can be explained by the mobility anisotropy of the A phase, which indicates that the vortices are continuous, i.e., consist of the A phase even in the core. In finite magnetic fields, a qualitatively similar continuous vortex structure was found, a result compatible with the earlier nuclear magnetic resonance (NMR) experiments, but also a new vortex type with an anomalously high μ c was seen when the vortex sample was prepared “adiabatically.” The observed strong ion focusing indicates broken w-symmetry in the core textures. This is in contradiction with the earlier numerical calculations on the optimal continuous vortex texture. Experiments to observe trapping of ions into the vortex cores were performed both in A and B phases. No trapping could be detected. Mobility measurements in stationary liquid were extended down toT=0.3T c . Comparison with the calculated transport coefficients yieldsΔ A (T)=1.32Δ BCS (T) for the maximal A phase gap atP=29.3 bar, andΔ B (T)=1.12Δ BCS (T) for the B phase gap at 4.8 bar. In the normal Fermi liquid a shallow minimum of ion mobility was detected aroundT≃2T c =5 mK.

First experiments suggesting the presence of singular vortices in rotating

^{3}\mathit{A}

are reported. Our studies on the interaction between negative ions and vortex textures verify the existence of the continuous vortices seen earlier by NMR, but evidence for a new vortex state with high ion mobility along the core is also found. The nucleation of the new vortex depends on the external magnetic field and the rotation history of the sample.

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.

A satellite peak has been observed in the NMR spectrum of rotating

^{3}\mathrm{He}

-

A

; the peak intensity depends linearly on $\ensuremath{\Omega}$ at the high angular velocities, $\ensuremath{\Omega}=0.6\ensuremath{-}1.5$ rad/s, needed to resolve it. The frequency shift of the satellite is independent of $\ensuremath{\Omega}$. These results strongly suggest the existence of vortices in rotating

^{3}\mathrm{He}

-

A

with the vortex density proportional to $\ensuremath{\Omega}$. Another satellite peak also has been observed which probably is due to solitons.