Far-infrared optical conductivity of NbN1−xCx thin films

Abstract Optical conductivity spectra σ ( ω )= σ 1 ( ω )+i σ 2 ( ω ) of NbN 1− x C x thin films have been measured in the far-infrared region at temperatures above and below T c , and have been analyzed based on the Mattis–Bardeen theory. The results suggest that NbN 1− x C x is a typical dirty-limit BCS-type superconductor in the strong-coupling regime. The value of the superconducting energy gap and the London penetration depth were estimated from optical results, which agree well with values reported previously and obtained from the electrical measurements.