Far-infrared thermal spectroscopy of low-Tc and high-Tc superconductor films
Temperature dependence of far-infrared transmission of NbN thin films deposited on MgO and Si substrates was measured at several frequencies from 0.4 to 4.3 THz. Activated exponential increase of relative penetration depth at low temperatures and a peak in transmission near Tc were observed for frequencies below the optical gap. On the other hand, the transmission measured at frequencies above the gap exhibits only flat, almost linear temperature dependence. This behaviour is consistent with the BCS theory of superconductivity. Similar measurements were performed also on YBa2Cu3O7−δ thin films deposited on MgO and sapphire substrates. Low-temperature variation of transmission indicates the d-wave symmetry. The peak below Tc predicted by the BCS theory is not observed. Flat temperature dependence of transmission at higher frequencies shows that the photon energy was sufficient for excitation over the optical gap. The s-wave BCS theory is adequate for NbN films but not for the YBa2Cu3O7−δ materials. Using the theoretical BCS model we show that the transmission peak is not correlated to the coherence peak in real conductivity at given frequency and other parameters relevant to experiment.