Dynamic fluctuations in the superconductivity ofNbNfilms from microwave conductivity measurements

We have measured the frequency and temperature dependences of complex ac conductivity, σ(ω)=σ₁(ω)-iσ₂(ω), of NbN films in zero magnetic fields between 0.1 and 100.3em0exGHz using a microwave broadband technique. In the vicinity of superconducting critical temperature Tc both σ₁(ω) and σ₂(ω) showed a rapid increase in the low frequency limit owing to the fluctuation effect of superconductivity. For the films thinner than 3000.3em0exnm, frequency and temperature dependences of fluctuation conductivity σfl(ω,T) were successfully scaled onto one scaling function, which was consistent with the Aslamazov and Larkin model for two-dimensional (2D) cases. For thicker films, σfl(ω,T) data could not be scaled, but indicated that the dimensional crossover from three dimensions to 2D occurred as the temperature approached Tc from above. This provides a good reference of ac fluctuation conductivity for more exotic superconductors of current interest.