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 up to 4.3 THz. Exponential increase of relative penetration depth at low temperatures and a peak in transmission near Tc was observed for frequencies below the optical gap. On the contrary, transmission measured at frequencies above the gap exhibits only flat, almost linear temperature dependence. This behavior is consistent with the BCS theory of superconductivity. Similar measurements were performed also on YBaCuO thin films deposited on MgO and sapphire substrates. Low-temperature variation of transmission is linear or constant and the peak below Tc predicted by the BCS theory is not observed. Flat temperature dependence at higher frequencies indicates 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 YBCO materials.