Optical study of superconducting Pr$_2$CuO$_{x}$ with $x\simeq 4$
  Superconducting Pr$_2$CuO$_x$, $x\simeq 4$ (PCO) films with $T^\prime$ structure and a $T_c$ of 27 K have been investigated by various optical methods in a wide frequency (7 - 55000 cm$^{-1}$) and temperature (2 to 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. 
G. Chanda, R. P. S. M. Lobo, E. Schachinger, J. Wosnitza, M. Naito,   and A. V. Pronin
cond-mat.supr-con cond-mat.str-el