Superconducting ${\mathrm{Pr}}_2$${\mathrm{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 $\left(\mathrm{7–55 000}{\mathrm{cm}}^{-1}\right)$ 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 $300{\mathrm{cm}}^{-1}$ 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 $1.6\mu \text{m}$, indicating a rather low density of the superconducting condensate.

• Received 27 January 2014
• Revised 17 June 2014

DOI:https://doi.org/10.1103/PhysRevB.90.024503