Kyung Ik Sim, Young Chan Jo, Taewoo Ha, Jong Hyeon Kim, Jae Hoon Kim, Hirotake Yamamori
https://link.springer.com/article/10.3938%2Fjkps.71.571
We have measured the transmission spectra of the conventional Bardeen-Cooper-Schrieffer (BCS) superconductor niobium nitride (NbN) thin films (Tc = 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, σ1, and the imaginary part, σ2, of the optical conductivity, σ̃ = σ1 + iσ2, independently and simultaneously, without a Kramers-Kronig analysis. The superconducting gap Δ(T) was observed in the real part of the conductivity, σ1, below Tc = 11 K with a maximum value of 2Δ(0) = 30 cm −1 and the gap ratio 2Δ(0)/kBTc = 3.92.
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