Sub-gap quasi-particle scattering and dissipation in superconducting Nb thin films

Niobium (Nb) thin films are used extensively in superconducting quantum circuits for quantum information science applications. We report on the terahertz time-domain spectroscopy (THz-TDS) experiments in which we measure the electrodynamics of superconducting (SC) Nb films at low temperatures in the GHz and THz spectral range. Above critical temperature (Tc), the thin film exhibits Drude-like response for both real and imaginary conductivity at normal state. Below Tc, the imaginary conductivity exhibits a diverging 1/ω response arising from SC condensate, whose conductivity can be derived from London equation. The real part of conductivity shows a quasi-particle peak at frequencies below, where the photon energy is insufficient to break a cooper pair. This sub-gap peak originates from impurity scattering different from thermally excited quasiparticles. Extended Drude model is performed to extract optical self-energy of quasiparticles, which provides information about impurity scattering, electron-phonon renormalization. Simulation of THz conductivity and quasi-particle scattering will be discussed and compared with our experiment data.