Direct observation of the superconducting gap in 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 perfomed on a 18 nm thick TiN film with a critical temperature of 3.4 K. Measurements were carried out from room temperature down to 2 K, and in magnetic fields up to 7 T. We extract the real and imaginary parts of the complex conductivity as a function of frequency and temperature, directly providing the superconducting energy gap. Further analysis yields the superconducting London penetration depth. The findings as well as the normal state properties strongly suggest conventional BCS superconductivity, underlined by the BCS ratio 3.44. Detailed analysis of the charge carrier dynamics of the silicon substrate is also discussed. 
Uwe S. Pracht and Marc Scheffler and Martin Dressel and David Kalok   and Cristoph Strunk and Tatyana I. Baturina
cond-mat.supr-con