Determining the depairing current in superconducting nanowire single-photon detectors

Abstract

We estimate the depairing current of superconducting nanowire single-photon detectors (SNSPDs) by studying the dependence of the nanowires' kinetic inductance on their bias current. The kinetic inductance is determined by measuring the resonance frequency of resonatorstyle nanowire coplanar waveguides both in transmission and reflection configurations. Bias current dependent shifts in the measured resonant frequency correspond to the change in the kinetic inductance, which can be compared with theoretical predictions. We demonstrate that the fast relaxation model described in the literature accurately matches our experimental data and provides a valuable tool for direct determination of the depairing current. Accurate and direct measurement of the depairing current is critical for nanowire quality analysis, as well as modeling efforts aimed at understanding the detection mechanism in SNSPDs. *

Multilayered Heater Nanocryotron: A Superconducting-Nanowire-Based Thermal Switch

“…Outside of this constricted area, the switching current is given by the temperature-dependent depairing current. These values lead to a constriction factor of approximately 0.58 ± 0.05 over the range of 0.4 ≤ T/T c ≤ 0.8, which is consistent with measurements of the switching current to depairing current ratio in thin NbN films [24]. It is important to note that the use of a constriction underneath the heater and a depairing current outside of the heater region is necessary to simultaneously reproduce the switching and retrapping characteristics of the device within the model.…”

Recent advances in superconducting nanowire single-photon detector technology for exoplanet transit spectroscopy in the mid-infrared

“…To quantify SNSPD quality, we used the constriction factor, which is the ratio of the current at which a nanowire ceases to have 0 resistance (the "switching current," which is limited by defects in the nanowire) to the maximum depairing current. 16 A constriction factor closer to 1 is desirable for sensitivity to low-energy photons. In that particular investigation, it was found that the constriction factor degraded for narrower superconducting strips (widths between 200 and 55 nm were studied), which suggests that the edge roughness plays a significant role in limiting the switching current.…”

Mid-infrared photon counting with superconducting nanowires

“…This is key for smaller energy mid-infrared photons when the effective hot-spot will be smaller as the deposited energy is smaller. Minimising wire width comes with problems, however, as narrower nanowires are more prone to fabrication imperfections, limiting the fraction of the depairing current that can be reached [109], which will in turn limit the timing jitter and detection efficiency. To avoid current-crowding effects in the bends of the nanowire it is also important to optimise the shape of the bends [110], ensuring rounded edges rather than abrupt corners.…”

“…Nanowire imperfections introduce position-dependent bias currents which makes the transition broader and in some cases can suppress the ability to bias the nanowire sufficiently enough to achieve saturated detection efficiency. Frasca et al [109] developed a method to determine the depairing current for each geometry and film used in each fabrication run and propose that the ratio of switching to depairing currents, C = I sw =I dep , is a more useful metric than critical current alone when determining device performance as it gives a measure of the quality of the nanowire. Maximising C will ensure good nanowire quality which will make it easier to bias at >I det across the entire nanowire ensuring saturated detection efficiency and minimimal timing jitter.…”