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Superconducting niobium nitride films are ideal candidates for electronics applications among low-Tc materials due to comparatively high critical temperature, large critical magnetic fields and relative ease of fabrication. The superconducting properties of NbN are known to be strongly dependent on the formation of a correct crystallographic phase. The tunneling spectroscopy data for NbN films are in good agreement with existing theoretical concepts, while contact methods provide significantly lower values of the superconducting order parameter. In this paper, we present corresponding results for a representative NbN film with Tc about 14 K, obtained using a point contact made by bringing a sharp metallic tip of silver in touch with the sample surface. We have observed fundamental difference between the gap values extracted from the differential conductance measurements and those following from the standard BCS theory. Despite the almost perfect contact of the normal tip with the film surface, the energy gap was nearly two times smaller than theoretically expected. At the same time, the gap values did not vanish at appreciably lower temperatures compared to the bulk of the NbN film, but rather continued to decrease slowly up to the bulk critical temperature. This finding is explained by significant deformation of the near-surface layer leading to the local suppression of superconducting parameters. Such behavior is a typical example of the superconducting proximity effect when the temperature dependence of the smaller gap strongly deviates from the BCS prediction but remains however finite up to the critical temperature of the larger gap, which is the same for two contacting spaces with superconducting parameters that differ significantly from each other. This conclusion is important for practical applications, in particular, for creating integration circuits based on superconducting NbN thin layers. [ABSTRACT FROM AUTHOR]

Superconducting quantum interference filter (SQIF) is an array of superconducting quantum interference devices (SQUIDs) with different loop sizes. Such a device is capable of providing a much higher transfer factor and much higher magnetic field sensitivity with respect to those of single SQUID. Series niobium nitride (NbN) SQIF containing 400 SQUIDs with randomly varied loop areas and epitaxial NbN/AlN/NbN Josephson junctions have been developed, fabricated, and measured at helium temperature. The device shows a nonperiodic voltage response with a single sharp peak at zero magnetic field. The measured magnetic flux to voltage trans fer factor reaches 37.8 mV/Ф0 at 4.2 K. [ABSTRACT FROM AUTHOR]

We fabricated superconducting nanowire single-photon detectors (SNSPDs) made of NbTiN and NbN thin films in the same fabrication batch. Various parameters, such as critical temperature, resistivity, switching current, and kinetic inductance, were compared between the two types of SNSPDs. These data indicate that NbTiN SNSPD surpasses NbN SNSPD in many aspects, including a shorter recovery time due to smaller kinetic inductance, a lower timing jitter due to higher switching current, and better uniformity. Based on the measurement of 28 out of 121 NbTiN SNSPDs on a 2-in Si wafer, 71% of detectors show a system detection efficiency of over 50% at a dark count rate of 100 Hz, with the maximum efficiency being 77.5%. [ABSTRACT FROM AUTHOR]

We report the characterization of optical coupling efficiency and the absorption coefficient of fiber-coupled superconducting nanowire single photon detector with an optical cavity, which limit the system detection efficiency. The small lenses were applied to compact fiber-coupled package and the efficient optical coupling higher than 95% were confirmed experimentally. The absorption coefficient was also verified and reached to \sim60% at 1550 nm wavelength and optimum polarized condition. The system detection efficiencies at 1550, 1310 and 810 nm were 28, 40 and 3%, respectively, which are related to the absorptance dependencies on wavelengths. [ABSTRACT FROM AUTHOR]

We report the evolution of superconducting properties with disorder, in 3-dimensional homogeneously disordered epitaxial NbN thin films. The effective disorder in NbN is controlled from moderately clean limit down to Anderson metal-insulator transition by changing the deposition conditions. We propose a phase diagram for NbN in temperature-disorder plane. With increasing disorder, we observe that as k l→1 the superconducting transition temperature ( T) and normal state conductivity in the limit T→0 ( σ) go to zero. The phase diagram shows that in homogeneously disordered 3-D NbN films, the metal-insulator transition and the superconductor-insulator transition occur at a single quantum critical point, k l∼1. [ABSTRACT FROM AUTHOR]