Extra-Drude contribution to the far infrared conductivity of oxygen deficient YBCO films

We measured reflectance spectra R(v) of oxygen deficient Y1Ba2Cu3O7-x (YBCO) films of different thickness, with critical temperatures between 85 and 87 K. Measurements were made at room temperature between 15 and 25000 cm-1. The measured R(v) have a maximum below 100 cm-1 and then decrease for v --> 0. This low frequency behaviour of R(v) can be accounted for by considering a relevant extra-Drude contribution to the far infrared optical conductivity of YBCO.