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 (similar to 10 MHz) response in long, superconducting NbN nanowires (100 nm wide, 4 nm thick, and 500 mu 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.