OPTICAL CONDUCTIVITY OF Nd 1.85Ce 0.15CuO 4: STRENGTH OF THE CONDENSATE

Abstract

The ab-plane reflectance of a Nd 1.85Ce 0.15CuO 4 single crystal ( T c =23 K) has been measured from ≈30 to 9500 cm −1 above and below T c , and the optical properties calculated from a Kramers–Kronig analysis. A rich phonon spectrum is observed with some new spectral features appearing below ≈100 K. The normal-state optical conductivity may be described by a Drude-like component and an overdamped mid infrared component. Below T c the plasma frequency of the condensate is ω pS ≈10 000 cm −1. The London penetration depth is determined to be λ ab =1600±100 Å, which places it well off the Uemura line. Estimates of the electron–phonon coupling from normal-state transport measurements yield λ tr <0.5. 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 to that in other hole-doped superconducting cuprates.