Macroscopic quantum tunnelling of vortices in He II.

Experimental evidence is presented showing that the creation of vortices in He II involves macroscopic quantum tunneling through, or thermal activation over, a potential-energy barrier E. The investigations, based on measurements of the rate at which negative ions create vortex rings in the isotopically pure superfluid within the temperature range 50 < T < 500 mK, have yielded a barrier height of E/kB = 3.1 ± 0.1 K, in excellent agreement with the theoretical value predicted by Muirhead, Vinen, and Donnelly