| Nucleation and pinning of vortex lines, as well as their interplay, have been long-standing problems in superfluid 4He. Progress has been made on the former subject during the recent years by studying phase slippage events of superfluid flow in submicron apertures. A consistent picture of vortex nucleation, either thermally activated or via quantum tunneling, as a half-ring at the wall has emerged from these sophisticated experiments performed most notably at Saclay (Paris). |
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We have elaborated on the
latter problem, the question of vortex pinning [PRL 81, 3451
(1998)]. According to Feynman the unpinning velocity
for vortices is inversely proportional to the size of the
flow geometry. Therefore, unpinning events on
nanometric structures are rather difficult to
distinguish from real nucleation events.
In our experiments (done in Saclay), however, such a distinction has been observed very clearly. This feature has allowed us to investigate quantitatively pinning/unpinning events of a single vortex line in superfluid 4He. From a study of the lifetime and unpinning velocities of the pinned vortices, we are able to conclude that unpinning takes place by quantum tunneling at the lowest temperatures. |
| FIGURE A possible configuration for a pinned vortex, giving rise to a large change in the apparent critical velocity for nucleation. When the flow direction, marked by straight arrows, comes from top-right corner toward the conical asperity featuring the nucleation site, the vortex inflates upward, away from the nucleation site. When the applied flow reverses and goes from the left bottom corner toward the nucleation site, the curvature of the pinned vortes gets inverted. The vortex curves downward, closer to the nucleation site. The velocity that it induces on the nucleation site adds to that of the applied flow and influences strongly the nucleation process. |