摘要：

We report on the first comprehensive measurements of critical superflow velocities in 3 He-B which allow different mechanisms of vortex formation to be identified. As a function of temperature T and pressure P , we measure the critical angular velocity Ω c (T, P) at which vortices start to form in slowly accelerating rotation in a cylindrical container filled with 3 He-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 v c (T, P)=Ω c (T, P), where R is the radius of the container. We find v c (T, P) to be related to the calculated intrinsic stability limit v 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.

展开