Optical study of superconducting Pr₂CuO_x with x similar or equal to 4

Superconducting Pr₂CuO_x, x similar or equal to 4 (PCO), films with T' structure and a T_c of 27 K have been investigated by various optical methods in a wide frequency (7 - 55 000 cm^-1) and temperature (2 - 300 K) range. The optical spectra do not reveal any indication of a normal-state gap formation. A Drude-like peak centered at zero frequency dominates the optical conductivity below 150 K. At higher temperatures, it shifts to finite frequencies. The detailed analysis of the low-frequency conductivity reveals that the Drude peak and a far-infrared (FIR) peak centered at about 300 cm^-1 persist at all temperatures. The FIR-peak spectral weight is found to grow at the expense of the Drude spectral weight with increasing temperature. The temperature dependence of the penetration depth follows a behavior typical for d-wave superconductors. The absolute value of the penetration depth for zero temperature is 1.6 mu m, indicating a rather low density of the superconducting condensate.