Terahertz electrodynamics and superconducting energy gap of NbN

We have measured the transmission spectra of the conventional Bardeen-Cooper-Schrieffer (BCS) superconductor niobium nitride (NbN) thin films ( T c = 11 K) using terahertz time-domain spectroscopy (THz-TDS) over the spectral range of 10 - 110 cm −1 and the temperature range of 3.9 - 295 K. We extracted both the real part, σ 1 , and the imaginary part, σ 2 , of the optical conductivity, σ̃ = σ 1 + iσ 2 , independently and simultaneously, without a Kramers-Kronig analysis. The superconducting gap Δ(T) was observed in the real part of the conductivity, σ 1 , below T c = 11 K with a maximum value of 2Δ(0) = 30 cm −1 and the gap ratio 2Δ(0)/ k B T c = 3.92.