Direct observation of the superconducting gap in a thin film of titanium nitride using terahertz spectroscopy

Abstract

We report on the charge carrier dynamics of superconducting titanium nitride (TiN) in the frequency range 90--510 GHz (3--17 cm${}^{\ensuremath{-}1}$). The experiments were performed on an 18-nm thick TiN film with a critical temperature of ${T}_{c}=3.4$ K. Measurements were carried out from room temperature down to 2 K, and in magnetic fields up to $B=7$ T. We extract the real and imaginary parts of the complex conductivity $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\ensuremath{\sigma}}$ as a function of frequency and temperature, directly providing the superconducting energy gap $2\ensuremath{\Delta}$. Further analysis yields the superconducting London penetration depth ${\ensuremath{\lambda}}_{L}$. The findings as well as the normal-state properties strongly suggest conventional BCS superconductivity, underlined by the ratio $2\ensuremath{\Delta}(0)/{k}_{B}{T}_{c}=3.44$. Detailed analysis of the charge carrier dynamics of the silicon substrate is also discussed.