NbN materials development for practical superconducting devices

Power switches such as a Superconducting Fault Current Limiter require large cross sectional area superconductors with both high critical current density Jcand normal state resistivity ρn. Large values of Jcand ρnhave been previously reported in small cross sectional area "weak links" of NbN. We report on reactively sputtered NbN films up to 5 μm thick and 2.2 cm wide which have ρn> 200 μΩ cm and a self-field Jcup to 10⁶A/cm². Severe degradation in Jcwas observed with increasing film width and for millisecond current pulses. This degradation could be substantially reduced by stabilization with either low ρnnormal metal or the use of a sapphire substrate. The resistivity and critical current dependence both imply Josephson coupled grains and the results will be discussed within that model.