Current-assisted thermally activated flux liberation in ultrathin nanopatterned NbN superconducting meander structures

Abstract

We present results from an extensive study of fluctuation phenomena in superconducting nanowires made from sputtered NbN. Nanoscale wires were fabricated in form of a meander and operated at a constant temperature ) 0 ( 4 . 0 c T T ≈ . The superconducting state is driven close to the electronic phase transition by a high bias current near the critical one.Fluctuations of sufficient strength temporarily drive a section of the meander structure into the normal conducting state, which can be registered as a voltage pulse of nanosecond duration.We considered three different models (vortex-antivortex pairs, vortex edge barriers and phase slip centers) to explain the experimental data. Only thermally excited vortices, either via unbinding of vortex-antivortex pairs or vortices overcoming the edge barrier, lead to a satisfactory and consistent description for all measurements. c T . At lower temperatures the probability of thermodynamic fluctuations drops exponentially so that they are experimentally no longer observable far below the transition temperature. However, the freezing-out of thermal fluctuations opens up the possibility to observe quantum fluctuations that prevail in the limit 0 = T , for example quantum phase-slips [16].Although well-defined one-and two-dimensional systems have been studied in great detail, the cross-over region between these limiting cases is less understood. This situation is just beginning to change as the size of superconducting conduction paths of devices such as SQUIDs or quantum detectors is continually decreasing, and therefore a better understanding of superconducting structures that are in between the limiting dimensions is required.I and are sensitive in the visible and near-infrared spectral range (3.1 -0.4 eV). It is generally believed that fluctuations are the major source of dark-count events in these detectors [22][23][24]. Measuring the dark-count rate thus gives us direct information about the fluctuation rates in a part of the superconducting phase diagram that is otherwise not easily accessible. The commonly used approach [23] to measure the DC resistance that is then used to infer the fluctuation rate is not appropriate at large bias currents close to the experimental critical current c,e I , since the Joule heating cannot be eliminated. By contrast, Joule heating may influence the amplitude and duration of individual voltage transients in our time-resolved measurements, but it does not superconducting systems [5]

Numerical analysis of detection-mechanism models of superconducting nanowire single-photon detector

“…Detector noise or so called dark counts increase approximately exponentially on approaching the experimental critical current. Experimental 25 and theoretical investigations 26 have favored magnetic vortices crossing the superconducting strips as the dominant mechanism leading to intrinsic darkcount events. Except near the ends of the straight sections of the superconducting meander the current density in the undisturbed equilibrium situation is homogeneous due to the fact that the strip width w ( K ¼ 2k 2 GL =d, where K is the effective 2D magnetic penetration depth, k GL ) d is the corresponding Ginzburg-Landau (GL) magnetic penetration depth in the bulk material and d the film thickness.…”

Tantalum nitride superconducting single-photon detectors with low cut-off energy

“…To date the majority of SNSPD have been made from NbN thin films due to their favorable characteristics. Superconducting NbN films with a T c ≈ 15 K can be made a few nanometer thin 2 and the resulting films can be structured down to strip widths of a few tens of nanometers 3,4 without destruction of superconductivity. According to simple detection models, 5 the threshold energy for direct detection decreases for materials with shorter thermalization time τ th , small electron-diffusion coefficient D, low density of states at the Fermi energy N 0 and small superconducting energy gap ∆.…”

Superconducting nanowire single photon detector on diamond

“…In SNSPDs, even when the input optical terminal is fully blocked, randomly distributed signals are generated at the output electrical terminal of the detector for a particular current bias condition. Regardless of the nature of these thermal fluctuations, the rate always obeys the Kramers formula [24][25][26] which predicts an exponential dependence of the rate to the height of the potential barrier faced by these thermal fluctuations. Since this potential barrier is current dependent, a power law dependence is predicted for the dark count versus bias current measurement as observed in Fig.2.…”