Low-frequency phase locking in high-inductance superconducting nanowires

Niobium nitride nanowires show considerable promise as high-speed single-photon detectors. We report the observation of an anomalous low-frequency $(\sim 10\mathrm{MHz})$ response in long, superconducting NbN nanowires ($100\mathrm{nm}$ wide, $4\mathrm{nm}$ thick, and $500\mu \mathrm{m}$ long). This behavior, although strikingly reminiscent of the ac Josephson effect, can be explained by a relaxation oscillation resulting from the high kinetic inductance of the type II nanowire. We simulate all of the observed effects using a simple resistive-hotspot/series-inductor model. The voltage pulses observed are indistinguishable from the pulses induced by visible photons, and our observations suggest noise-induced relaxation oscillations are one mechanism for the dark counts in photon detectors.