Critical velocity of continuous vortex nucleation in a slab of superfluid 3He-A -: art. no. 225301

A flow-induced Freedericksz transition is observed in a 0.26 mm thick disk-shaped slab of superfluid He-3-A using a rotating cryostat and a torsional oscillator, and it is used to detect vortices in zero magnetic field. The phenomena are studied as a function of magnetic field normal to the slab. In defect-free (l) over cap texture the critical velocity for vortex nucleation is 0.5 mm/s, but in the presence of a domain wall it is reduced to similar to(h) over bar /2ma(c), where a(c)(H) is the field-dependent radius of the vortex soft core. The vortices nucleate at a distance at least 0.3 mm from the outer edge of the disk.