Optical properties of Nd1.85Ce0.15CuO4

The ab-plane reflectance of a Nd 1.85Ce 0.15CuO 4 single crystal (Tc523 K! has been measured from '35 to 9500 cm 21 at temperatures above and below Tc , 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 Drudelike component and an overdamped midinfrared component. The Drude-like component narrows rapidly; at 30 K the mean free path is estimated to be '750 Å. Below Tc the Drude carriers collapse into the condensate; the plasma frequency of the d function is determined to be vpS'10 000 cm . The small coherence length (j0'70 Å! places this material into the clean limit (j0 /l!1). The London penetration depth is determined to be lab516006100 Å, which places it well off the Uemura line. Estimates of the electron-phonon coupling from normal-state transport measurements of l tr,0.5, and the absence of Holstein sidebands below Tc , 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. @S0163-1829~97!00434-7#