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Zolotov, P. I., Semenov, A. V., Divochiy, A. V., Goltsman, G. N., Romanov, N. R., & Klapwijk, T. M. (2021). Dependence of photon detection efficiency on normal-state sheet resistance in marginally superconducting films of NbN. IEEE Trans. Appl. Supercond., 31(5), 1–5.
Abstract: We present an extensive set of data on nanowire-type superconducting single-photon detectors based on niobium-nitride (NbN) to establish the empirical correlation between performance and the normal-state resistance per square. We focus, in particular, on the bias current, compared to the expected depairing current, needed to achieve a near-unity detection efficiency for photon detection. The data are discussed within the context of a model in which the photon energy triggers the movement of vortices i.e. superconducting dissipation, followed by thermal runaway. Since the model is based on the non-equilibrium theory for conventional superconductors deviations may occur, because the efficient regime is found when NbN acts as a marginal superconductor in which long-range phase coherence is frustrated.
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Samsonova, A. S., Zolotov, P. I., Baeva, E. M., Lomakin, A. I., Titova, N. A., Kardakova, A. I., et al. (2021). Signatures of surface magnetic disorder in niobium films. IEEE Trans. Appl. Supercond., 31(5), 1–5.
Abstract: We present our studies on the evolution of the normal and superconducting properties with thickness of thin Nb films with a low level of non-magnetic disorder ( kFl≈150 for the thickest film in the set). The analysis of the superconducting behavior points to the presence of magnetic moments, hidden in the native oxide on the surface of Nb films. Using the Abrikosov-Gorkov theory, we obtain the density of surface magnetic moments of 1013 cm −2 , which is in agreement with the previously reported data for Nb films.
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Shangina, E. L., Smirnov, K. V., Morozov, D. V., Kovalyuk, V. V., Goltsman, G. N., Verevkin, A. A., et al. (2011). Concentration dependence of energy relaxation time in AlGaAs/GaAs heterojunctions: direct measurements. Semicond. Sci. Technol., 26(2), 025013.
Abstract: We present measurements of the energy relaxation time, τε, of electrons in a single heterojunction in a quasi-equilibrium state using microwave time-resolved spectroscopy at 4.2 K. We find the relaxation time has a power-law dependence on the carrier density of the two-dimensional electron gas, τε∝nγs with γ = 0.40 ± 0.02 for values of the carrier density, ns, from 1.6 × 1011 to 6.6 × 1011cm−2. The results are in good agreement with predictions taking into account the scattering of the carriers by both piezoelectric and deformation potential acoustic phonons. We compare these results with indirect measurements of the energy relaxation time from energy loss measurements involving Joule heating of the electron gas.
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Zhang, W., Miao, W., Zhong, J. Q., Shi, S. C., Hayton, D. J., Vercruyssen, N., et al. (2014). Temperature dependence of the receiver noise temperature and IF bandwidth of superconducting hot electron bolometer mixers. Supercond. Sci. Technol., 27(8), 085013 (1 to 5).
Abstract: In this paper we study the temperature dependence of the receiver noise temperature and IF noise bandwidth of superconducting hot electron bolometer (HEB) mixers. Three superconducting NbN HEB devices of different transition temperatures (Tc) are measured at 0.85 THz and 1.4 THz at different bath temperatures (Tbath) between 4 K and 9 K. Measurement results demonstrate that the receiver noise temperature of superconducting NbN HEB devices is nearly constant for Tbath/Tc, less than 0.8, which is consistent with the simulation based on a distributed hot-spot model. In addition, the IF noise bandwidth appears independent of Tbath/Tc, indicating the dominance of phonon cooling in the investigated HEB devices.
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Galin, M. A., Klushin, A. M., Kurin, V. V., Seliverstov, S. V., Finkel, M. I., Goltsman, G. N., et al. (2015). Towards local oscillators based on arrays of niobium Josephson junctions. Supercond. Sci. Technol., 28(5), 055002 (1 to 7).
Abstract: Various applications in the field of terahertz technology are in urgent need of compact, wide-tunable solid-state continuous wave radiation sources with a moderate power. However, satisfactory solutions for the THz frequency range are scarce yet. Here we report on coherent radiation from a large planar array of Josephson junctions (JJs) in the frequency range between 0.1 and 0.3 THz. The external resonator providing the synchronization of JJ array is identified as a straight fragment of a single-strip-line containing the junctions themselves. We demonstrate a prototype of the quasioptical heterodyne receiver with the JJ array as a local oscillator and a hot-electron bolometer mixer.
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