Optical properties ofNd1.85Ce0.15CuO4

Abstract

The ab-plane reflectance of a Nd{sub 1.85}Ce{sub 0.15}CuO{sub 4} single crystal (T{sub c}=23 K) has been measured from {approx}35 to 9500 cm{sup {minus}1} at temperatures above and below T{sub 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 midinfrared component. The Drude-like component narrows rapidly; at 30 K the mean free path is estimated to be {approx}750 {Angstrom}. Below T{sub c} the Drude carriers collapse into the condensate; the plasma frequency of the {delta} function is determined to be {omega}{sub pS}{approx}10000 cm{sup {minus}1}. The small coherence length ({xi}{sub 0}{approx}70 {Angstrom}) places this material into the clean limit ({xi}{sub 0}/l{lt}1). The London penetration depth is determined to be {lambda}{sub ab}=1600{plus_minus}100 {Angstrom}, which places it well off the Uemura line. Estimates of the electron-phonon coupling from normal-state transport measurements of {lambda}{sub tr}{lt}0.5, and the absence of Holstein sidebands below T{sub c}, indicate that the carriers that participatemore » 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. {copyright} {ital 1997} {ital The American Physical Society}« less