Switching currents limited by single phase slips in one-dimensional superconducting Al nanowires

An aluminum nanowire switches from superconducting to normal as the current is increased in an upsweep. The switching current (I_s) averaged over upsweeps approximately follows the depairing critical current (I_c) but falls below it. Fluctuations in I_s exhibit three distinct regions of behaviors and are nonmonotonic in temperature: saturation well below the critical temperature T_c , an increase as T^2/3 at intermediate temperatures, and a rapid decrease close to T_c . Heat dissipation analysis indicates that a single phase slip is able to trigger switching at low and intermediate temperatures, whereby the T^2/3 dependence arises from the thermal activation of a phase slip, while saturation at low temperatures provides striking evidence that the phase slips by macroscopic quantum tunneling.