UPPER CRITICAL-FIELD OF NBN FILM

It is proposed and experimentally verified that the anomalously high superconducting critical field normal to the surface of NbN films possessing a column-void microstructure is Hc3, the field appropriate for surface superconductivity. It is also proposed that because the coherence length is much less than the lateral column dimension that the resistivity of the column and not the film enters calculations of the Maki parameter α. A previously noted discrepancy in α is removed by these proposals.