Optical determination of the superconducting energy gap in electron-doped Pr_(1.85)Ce_(0.15)CuO₄

The optical properties of single crystal Pr_{1.85}Ce_{0.15}CuO_4 have been measured over a wide frequency range above and below the critical temperature (T_c \simeq 20 K). In the normal state the coherent part of the conductivity is described by the Drude model, from which the scattering rate just above T_c is determined to be 1/\tau \simeq 80 cm^{-1}. The condition that \hbar/\tau \approx 2k_B T near T_c appears to be a general result in many of the cuprate superconductors. Below T_c the formation of a superconducting energy gap is clearly visible in the reflectance, from which the gap maximum is estimated to be \Delta_0 \simeq 35 cm^{-1} (4.3 meV). The ability to observe the superconducting energy gap in the optical properties favors the nonmonotonic over the monotonic description of the d-wave gap. The penetration depth for T\ll T_c is \lambda \simeq 2000 \AA, which when taken with the estimated value for the dc conductivity just above T_c of \sigma_{dc} \simeq 35 \times 10^3 \Omega^{-1}cm^{-1} places this material on the general scaling line for the cuprates defined by 1/\lambda^2 \propto \sigma_{dc}(T\simeq T_c) \times T_c. This result is consistent with the observation that 1/\tau \approx 2\Delta_0, which implies that the material is not in the clean limit.