Terahertz time‐domain measurement of the conductivity and superconducting band gap in niobium

We measure the complex conductivity of thin superconducting niobium films using coherent time‐domain terahertz spectroscopy. Both real and imaginary part of the conductivity are measured simultaneously without referring to the Kramers–Kronig relations. We also measure the superconducting band gap directly from the onset of absorption and compare our results with theory.