DEPENDENCE OF THE UPPER CRITICAL FIELD AND CRITICAL CURRENT ON RESISTIVITY IN NbN THIN FILMS.

In this study we have measured H//c//2(T) and J//c(4. 2K) on a series of NbN thin films which have a fine (5 nm) equiaxed grain structure with no evidence for columnar voids. Samples with a range of critical temperatures and resistivities have been made by varying the percentage of nitrogen in argon during the deposition. The slope of the upper critical field at T//c saturates for rho greater than 250 mu OMEGA -cm (the calculated maximum resistivity for NbN). Despite the lack of columnar grain structure, H//c//2 is anisotropic, with H//c//2( PERPEND ) greater than H//c//2( parallel ). At low temperatures H//c//2(T) is limited by Pauli paramagnetism ( lambda //s//o approximately equals 5) which suggests that further increases in H//c//2(0) may be possible if additional spin-orbit scattering can be induced in the material. Finally, the critical currents for these very fine-grained samples are similar to the values found by others in larger columnar-grained samples.