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.