Parallel Critical Field in Thin Niobium Films: Comparison to Theory

For the first time, a comparison to the predicted behavior for parallel critical field is carried out for the model of Kogan and the model of Hara and Nagai. In this study, thin niobium films in the moderately dirty regime were considered. Experimental values of the $-C_{2}$ term are seen to be lower than those from the model of Hara and Nagai. A possible reason for this could be not including the non spherical Fermi surface of niobium into the model. There is clearly disagreement with the model of Kogan as the films get cleaner and thinner, and two films which should be below his critical thickness still show positive values of $-C_{2}$, in disagreement with his theory.