Intrinsic and extrinsic mechanisms of vortex formation in superfluid3He-B |
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Authors: | V M H Ruutu Ü Parts J H Koivuniemi N B Kopnin M Krusius |
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Affiliation: | (1) Low Temperature Laboratory, Helsinki University of Technology, 02150 Espoo, Finland;(2) Present address: Landau Institute for Theoretical Physics, 117334 Moscow, Russia |
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Abstract: | We report on the first comprehensive measurements of critical superflow velocities in3He-B which allow different mechanisms of vortex formation to be identified. As a function of temperatureT and pressureP, we measure the critical angular velocity Ωc(T, P) at which vortices start to form in slowly accelerating rotation in a cylindrical container filled with3He-B. Owing to the long coherence length ξ(T, P)∼10–100 nm, either trapped remanent vorticity or intrinsic nucleation may dominate vortex formation, depending on the roughness
of the container wall and the presence of loaded traps.
NMR measurement with a resolution of one single vortex line allows us to distinguish between different processes: (1) Three
extrinsic mechanisms of vortex formation have been observed. One of them is the vortex mill, a continuous periodic source
which is activated in a rough-walled container well below the limit for intrinsic nucleation. (2) In a closed smooth-walled
container intrinsic nucleation is the only mechanism available, with a critical velocity vc(T, P)=Ωc(T, P), whereR is the radius of the container. We findv
c
(T, P) to be related to the calculated intrinsic stability limitv
ch
(T, P) of homogeneous superflow. The existence of this connection in the form of a scaling law implies that nucleation takes place
at an instability, rather than by thermal activation or quantum tunneling which become impossible because of an inaccessibly
high energy barrier. |
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