EVIDENCE FOR QUANTUM TUNNELING OF PHASE-SLIP VORTICES IN SUPERFLUID HE-4

We have observed that at temperatures below 200 mK quantized vortices are created by a new process which is not described by the thermal activation theory of vortex nucleation. The critical flow velocity in a submicron orifice has been determined using two techniques. Down to about 200 mK the critical velocities rise linearly with failing temperature. Below this temperature the critical velocity stops rising and becomes almost temperature independent, indicating that a new process, possibly quantum tunneling, dominates the phase-slip nucleation.