Optical properties of Nd1.85Ce0.15CuO4

The nb-plane reflectance of a Nd1.85Ce0.15CuO4 single crystal (T-c=23 K) has been measured from approximate to 35 to 9500 cm(-1) at temperatures above and below T-c, and the optical properties calculated from a Kramers-Kronig analysis. A rich phonon spectrum is observed, and there are a number of c-axis infrared and Raman modes that are observed at low temperature which are believed to be activated by disorder; several of these modes show evidence for electron-phonon coupling. The normal-state optical conductivity may be described by a Drude-like component and an overdamped micinfrared component. The Drude-like component narrows rapidly; at 30 K the mean free path is estimated to be approximate to 750 Angstrom. Below T-c the Drude carriers collapse into the condensate; the plasma frequency of the delta function is determined to be omega(pS)approximate to 10 000 cm(-1). The small coherence length (xi(0) approximate to 70 Angstrom) places this material into the clean limit (xi(0)/l much less than 1). The London penetration depth is determined to be lambda(ab)-1600+/-100 Angstrom, which places it well off the Uemura line. Estimates of the electron-phonon coupling from normal-state transport measurements of lambda(tr)<0.5, and the absence of Holstein sidebands below T-c, indicate that the carriers that participate in superconductivity are weakly coupled to the phonons. The small values for the penetration depth and the electron-phonon coupling constant suggest that the superconductivity in this material is not due to the electron-phonon mechanism, and is different than in other hole-doped superconducting cuprates.