Far-infrared conductivity measurements of pair breaking in superconducting Nb 0.5 Ti 0.5 N thin films induced by an external magnetic field

We report the complex optical conductivity of a superconducting thin film of Nb 0.5 Ti 0.5 N in an external magnetic field. The field was applied parallel to the film surface and the conductivity extracted from far-infrared transmission and reflection measurements. The real part shows the superconducting gap, which we observe to be suppressed by the applied magnetic field. We compare our results with the pair-breaking theory of Abrikosov and Gor'kov and confirm directly the theory's validity for the optical conductivity.