15 May 2017 Superconducting order parameter fluctuations in NbN/NiCu and NbTiN/NiCu bilayer nanostripes for photon detection
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Thermodynamic fluctuations of the superconducting order parameter in NbN/NiCu and NbTiN/NiCu superconductor/ferromagnet (S/F) thin bilayers patterned to microbridges are investigated. Plain NbN and NbTiN films served as reference materials for the analyses. The samples were grown using dc-magnetron sputtering on chemically cleaned sapphire single-crystal substrates. After rapid thermal annealing at high temperatures, the superconducting films were coated with NiCu overlays, using co-sputtering. The positive magnetoresistance of the superconducting single layers is very small in the normal state but with a sharp upturn close to the superconducting transition, a familiar signature of superconducting fluctuations. The fluctuation-enhanced conductivity (paraconductivity) of the NbN and NbTiN single layer films is slightly larger than the prediction of the parameter-free Aslamazov-Larkin theory for order-parameter fluctuations in two-dimensional superconductors. The addition of a ferromagnetic top layer, however, changes the magnetotransport properties significantly. The S/F bilayers show a negative magnetoresistance up to almost room temperature, while the signature of fluctuations is similar to that in the plain films, demonstrating the relevance of both ferromagnetic and superconducting effects in the S/F bilayers. The paraconductivity is reduced below theoretical predictions, in particular in the NbTiN/NiCu bilayers. Such suppression of the fluctuation amplitude in S/F bilayers could be favorable to reduce dark counts in superconducting photon detectors and lead the way to enhance their performance.
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Bernd Aichner, Bernd Aichner, Florian Jausner, Florian Jausner, Georg Zechner, Georg Zechner, Rita Mühlgassner, Rita Mühlgassner, Wolfgang Lang, Wolfgang Lang, Andrii Klimov, Andrii Klimov, Roman Puźniak, Roman Puźniak, Wojciech Słysz, Wojciech Słysz, Marek Guziewicz, Marek Guziewicz, Renata Kruszka, Renata Kruszka, Maciej Węgrzecki, Maciej Węgrzecki, Roman Sobolewski, Roman Sobolewski, } "Superconducting order parameter fluctuations in NbN/NiCu and NbTiN/NiCu bilayer nanostripes for photon detection", Proc. SPIE 10229, Photon Counting Applications 2017, 102290H (15 May 2017); doi: 10.1117/12.2270877; https://doi.org/10.1117/12.2270877

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