Direct observation of the superconducting energy gap developing in the conductivity spectra of niobium

The electrodynamic response of Nb in the frequency range above and below the energy gap 2Δ is studied in the normal and in the superconducting state. A coherent source interferometer is utilized in the spectral range between 5 and 30 cm^-1 to investigate the amplitude and the phase of the transmission through niobium films that allows the direct determination of both components of the complex conductivity. The optical conductivity up to 300 cm^-1 is evaluated using infrared reflection measurements. Below the 8.31-K superconducting transition temperature of the 150-Å-thick film, the superconducting energy gap is observed to increase as the temperature decreases. The overall frequency dependence of the conductivity can be described using the BCS formalism and assuming finite scattering effects; however, at low temperatures we find deviations from the predicted behavior in the spectral range below the energy gap. Estimations of the gap at zero temperature 2Δ(0) = 24 cm^-1 or 4.1k_BT_c deviate from the weak-coupling BCS limit.