rf superconducting properties of reactively sputtered NbN

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Abstract
Rf properties of superconducting niobium nitride (NbN) have been studied for the first time by using C‐band (6.5 GHz) TE011 mode cavities. NbN films have been deposited on inner surfaces of the cavities made of Nb by the reactive sputtering method. From the temperature dependence of the surface resistance the superconducting energy gap of NbN has been determined. Although Tc remains high, the energy gap is apparently very small [2D(0)?1.7] or 2.0 kBTc. It recovers, however, to a value 2D(0)?2.7 or 2.8kBTc with a dc magnetic field applied parallel to the cavity axis. Measurements have been made on both rf and dc field dependences of the surface impedance. Different behaviors have been observed between Nb and NbN cavities. A columnar structure model well accounts for these behaviors of NbN films. The NbN cavity is more stable against several gases and more insensitive to trapped dc field than Nb. The field is, however, limited purely thermally at relatively low levels probably owing to the weak superconductivity around the columns and to the poor thermal conductivity of NbN.