Optical determination of the superconducting energy gap in electron-doped Pr1.85Ce0.15CuO4

The optical properties of single crystal Pr1.85Ce0.15CuO4 have been measured over a wide frequency range above and below the critical temperature (T-c similar or equal to 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 similar or equal to 80 cm(-1). The condition that h/tau approximate to 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)similar or equal to 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 < T-c is lambda similar or equal to 2000 A, which when taken with the estimated value for the dc conductivity just above T-c of sigma(dc)similar or equal to 35x10(3) Omega(-1) cm(-1) places this material on the general scaling line for the cuprates defined by 1/lambda(2)proportional to sigma(dc)(T similar or equal to T-c)center dot T-c. These results are consistent with the observation that 1/tau approximate to 2 Delta(0), which implies that the material is not in the clean limit.