Transport measurements in granular niobium nitride cermet films

We have studied normal-state and superconducting transport properties in a granular cermet consisting of B1-structure NbN grains in a boron nitride insulating matrix. By varying the volume fraction of the two components we produced films (20–70 nm in thickness) that exhibited transport behavior in either of two distinct and mutually exclusive classes: ‘‘insulating’’ films with ρ∼exp(-a/$T^{1/2}$) which never went superconducting, and ‘‘superconducting’’ films with ρ∼ln(T) from room temperature down to the superconducting transition. This latter logarithmic temperature dependence for the resistivity is also observed at low temperature when superconductivity is suppressed in high magnetic fields. Broad superconducting transitions are observed with a strong compositional dependence of the mean field critical temperature, $T_{c0}$. The Kosterlitz-Thouless two-dimensional (2D) topological phase transition at $T_{2\mathrm{D}}$ is observed in the superconducting samples both by the power-law behavior in current-voltage characteristics and by pure flux flow transport in high magnetic fields.