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Saveskul NA, Titova NA, Baeva EM, Semenov AV, Lubenchenko AV, Saha S, et al. Superconductivity behavior in epitaxial TiN films points to surface magnetic disorder. Phys Rev Applied. 2019;12(5):054001.
Abstract: We analyze the evolution of the normal and superconducting properties of epitaxial TiN films, characterized by high Ioffe-Regel parameter values, as a function of the film thickness. As the film thickness decreases, we observe an increase of the residual resistivity, that becomes dominated by diffusive surface scattering for d≤20nm. At the same time, a substantial thickness-dependent reduction of the superconducting critical temperature is observed compared to the bulk TiN value. In such high-quality material films, this effect can be explained by a weak magnetic disorder residing in the surface layer with a characteristic magnetic defect density of approximately 1012cm−2. Our results suggest that surface magnetic disorder is generally present in oxidized TiN films.
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Bulaevskii LN, Graf MJ, Kogan VG. Vortex-assisted photon counts and their magnetic field dependence in single-photon superconducting detectors. Phys Rev B. 2012;85(1):9.
Abstract: 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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Peltonen JT, Astafiev OV, Korneeva YP, Voronov BM, Korneev AA, Charaev IM, et al. Coherent flux tunneling through NbN nanowires. Phys Rev B. 2013;88(22):220506 (1 to 5).
Abstract: We demonstrate evidence of coherent magnetic flux tunneling through superconducting nanowires patterned in a thin highly disordered NbN film. The phenomenon is revealed as a superposition of flux states in a fully metallic superconducting loop with the nanowire acting as an effective tunnel barrier for the magnetic flux, and reproducibly observed in different wires. The flux superposition achieved in the fully metallic NbN rings proves the universality of the phenomenon previously reported for InOx. We perform microwave spectroscopy and study the tunneling amplitude as a function of the wire width, compare the experimental results with theories, and estimate the parameters for existing theoretical models.
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Korneeva Y P, Vodolazov D Y, Semenov A V, Florya I N, Simonov N, Baeva E, et al. Optical single-photon detection in micrometer-scale NbN bridges. Phys Rev Applied. 2018;9(6):064037 (1 to 13).
Abstract: We demonstrate experimentally that single-photon detection can be achieved in micrometer-wide NbN bridges, with widths ranging from 0.53 to 5.15 μm and for photon wavelengths of 408 to 1550 nm. The microbridges are biased with a dc current close to the experimental critical current, which is estimated to be about 50% of the theoretically expected depairing current. These results offer an alternative to the standard superconducting single-photon detectors, based on nanometer-scale nanowires implemented in a long meandering structure. The results are consistent with improved theoretical modeling based on the theory of nonequilibrium superconductivity, including the vortex-assisted mechanism of initial dissipation.
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Kozorezov AG, Lambert C, Marsili F, Stevens MJ, Verma VB, Stern JA, et al. Quasiparticle recombination in hotspots in superconducting current-carrying nanowires. Phys Rev B. 2015;92(6).
Abstract: We describe a kinetic model of recombination of non-equilibrium quasiparticles generated by single photon absorption in superconducting current-carrying nanowires. The model is developed to interpret two-photon detection experiments in which a single photon does not possess sufficient energy for breaking superconductivity at a fixed low bias current. We show that quasiparticle self- recombination in relaxing hotspot dominates diffusion expansion effects and explains the observed strong bias current, wavelength and temperature dependencies of hotspot relaxation in tungsten silicide superconducting nanowire single-photon detectors.
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