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

We report on the charge carrier dynamics of superconducting titanium nitride (TiN) in the frequency range 90-510 GHz (3-17 cm^-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 σ̂ as a function of frequency and temperature, directly providing the superconducting energy gap 2Δ. Further analysis yields the superconducting London penetration depth λ_L. The findings as well as the normal-state properties strongly suggest conventional BCS superconductivity, underlined by the ratio 2Δ(0)/k_BT_c=3.44. Detailed analysis of the charge carrier dynamics of the silicon substrate is also discussed.