Intrinsic and extrinsic mechanisms of vortex formation in superfluid He-3-B

We report on the first comprehensive measurements of critical superflow velocities in He-3-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 Omega(c)(T, P) at,which vortices start to form in slowly accelerating rotation in a cylindrical container filled with He-3-B. Owing to the long coherence length xi(T, P)similar to 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,vith a resolution of one single vortex line allows to 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)=Omega(c)(T, P) R, where R is the radius of the container. We find v(c)(T, P) to be related to the calculated intrinsic stability limit v(cb)(T, P) of homogeneous superflow. The existence of this connection in rite 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.