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arXiv:1206.4579 · Physical Review B

Dependence of count rate on magnetic field in superconducting thin-film TaN single-photon detectors

Andreas Engel, A. Schilling, K. Ilin, M. Siegel

Published Oct 22, 2012• 35 citations
arXivHTMLPDF

Abstract

We have studied the magnetic-field dependence of both dark-count rates and photon-count rates in a superconducting nanowire single-photon detector made of TaN in external magnetic fields $|{\ensuremath{\mu}}_{0}H|<10$ mT perpendicular to the plane of the underlying meander structure and at $T=4$ K. The dark-count rates show a characteristic field dependence, which is asymmetric with respect to magnetic-field direction. The field and the current dependence of the dark counts can be quantitatively well explained if one assumes that the critical current is reduced to $\ensuremath{\approx}$$50%$ at the ${180}^{\ensuremath{\circ}}$ meander turns when compared to a straight strip, and the observed asymmetry can be modeled assuming that the turnarounds are not all strictly equal. Surprisingly, the photon-count rates do not show any significant field dependence, which seems to be at odds with existing detection models invoking vortex crossings.

DOI
10.1103/physrevb.86.140506

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We have studied the magnetic-field dependence of both dark-count rates and photon-count rates in a superconducting nanowire single-photon detector made of TaN in external magnetic fields ∣\ensuremathμ0H∣<10|{\ensuremath{\mu}}_{0}H|<10∣\ensuremathμ0​H∣<10 mT perpendicular to the plane of the underlying meander structure and at T=4T=4T=4 K. The dark-count rates show a characteristic field dependence, which is asymmetric with respect to magnetic-field direction. The field and the current dependence of the dark counts can be quantitatively well explained if one assumes that the critical current is reduced to \ensuremath{\approx}$$50% at the 180\ensuremath∘{180}^{\ensuremath{\circ}}180\ensuremath∘ meander turns when compared to a straight strip, and the observed asymmetry can be modeled assuming that the turnarounds are not all strictly equal. Surprisingly, the photon-count rates do not show any significant field dependence, which seems to be at odds with existing detection models invoking vortex crossings.
We have studied the influence of an externally applied magnetic field on the photon and dark count rates of meander-type niobium nitride superconducting nanowire single-photon detectors. Measurements have been performed at a temperature of 4.2 K, and magnetic fields up to 250 mT have been applied perpendicularly to the meander plane. While photon count rates are field independent at weak applied fields, they show a strong dependence at fields starting from approximately \ifmmode\pm\else\textpm\fi{}25 mT. This behavior, as well as the magnetic field dependence of the dark count rates, is in good agreement with the recent theoretical model of vortex-assisted photon detection and spontaneous vortex crossing in narrow superconducting lines. However, the local reduction of the superconducting free energy due to photon absorption, which is the fitting parameter in the model, increases much slower with the photon energy than the model predicts. Furthermore, changes in the free-energy during photon counts and dark counts depend differently on the current that flows through the meander. This indicates that photon counts and dark counts occur in different parts of the meander.
We argue that photon counts in a superconducting nanowire single-photon detector (SNSPD) are caused by the transition from a current-biased metastable superconducting state to the normal state. Such a transition is triggered by vortices crossing the thin and narrow superconducting strip from one edge to another due to the Lorentz force. Detector counts in SNSPDs may be caused by three processes: (a) a single incident photon with sufficient energy to break enough Cooper pairs to create a normal-state belt across the entire width of the strip (direct photon count), (b) thermally induced single-vortex crossing in the absence of photons (dark count), which at high-bias currents releases the energy sufficient to trigger the transition to the normal state in a belt across the whole width of the strip, and (c) a single incident photon of insufficient energy to create a normal-state belt but initiating a subsequent single-vortex crossing, which provides the rest of the energy needed to create the normal-state belt (vortex-assisted single-photon count). We derive the current dependence of the rate of vortex-assisted photon counts. The resulting photon count rate has a plateau at high currents close to the critical current and drops as a power law with high exponent at lower currents. While the magnetic field perpendicular to the film plane does not affect the formation of hot spots by photons, it causes the rate of vortex crossings (with or without photons) to increase. We show that by applying a magnetic field one may characterize the energy barrier for vortex crossings and identify the origin of dark counts and vortex-assisted photon counts.
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We investigate the response mechanism of nanowire superconducting single-photon detectors (SSPDs) made of amorphous MoxSi1-x. We study the dependence of photon count and dark count rates on bias current in magnetic fields up to 113 mT at 1.7 K temperature. The observed behavior of photon counts is similar to the one recently observed in NbN SSPDs. Our results show that the detecting mechanism of relatively high-energy photons does not involve the vortex penetration from the edges of the film, and on the contrary, the detecting mechanism of low-energy photons probably involves the vortex penetration from the film edges.
We use an external magnetic field to probe the detection mechanism of a superconducting nanowire single-photon detector. We argue that the hot belt model (which assumes partial suppression of the superconducting order parameter $\mathrm{\ensuremath{\Delta}}$ across the whole width of the superconducting nanowire after absorption of the photon) does not explain observed weak-field dependence of the photon count rate (PCR) for photons with $\ensuremath{\lambda}=450\phantom{\rule{4.pt}{0ex}}\mathrm{nm}$ and noticeable decrease of PCR (with increasing the magnetic field) in a range of the currents for photons with wavelengths $\ensuremath{\lambda}=450--1200\phantom{\rule{4.pt}{0ex}}\mathrm{nm}$. Found experimental results for all studied wavelengths can be explained by the vortex hot spot model (which assumes partial suppression of $\mathrm{\ensuremath{\Delta}}$ in the area with size smaller than the width of the nanowire) if one takes into account nucleation and entrance of the vortices to the photon induced hot spot and their pinning by the hot spot with relatively large size and strongly suppressed $\mathrm{\ensuremath{\Delta}}$.
Abstract In a magnetic field, vortices significantly contribute to the dark counts of single-photon detectors made of superconducting wires, and they are also limiting the critical current of such devices. To address this issue, we prepared superconducting microwires with a pinhole grid from WSi thin films and report on corresponding critical-current and count-rate measurements in an external magnetic field B. When compared to corresponding devices without pinholes, the critical current only weakly depends on the magnetic field at B < 16 mT and it is even larger already at B > 10 mT. Moreover, dark counts are not only suppressed in zero field, but particularly in magnetic fields B < 16 mT, while photon counts are virtually field insensitive in the same range of the magnetic field.