Mid-infrared reflectivity of single crystal Nd_2-xCe_xCu₄O_4-y

We have measured infrared reflectivity of single crystals of Nd_2-xCe_xCu₄O_4-y in the range between 0.05 eV and 1.5 eV. The superconducting or nearly superconducting crystals studied show a very high mid-IR reflectivity. Reflectivity of the samples is fitted much better with a Drude model, which yielded very similar fitting parameters (ω_p ∼ 2.5 eV) for all samples, than YBa₂Cu₃O₇ or Bi₂Sr₂CuO₆. However, a deviation from a Drude model was evident in the very low frequency and the fitting was considerably improved with a Drude-Lorentz model in both fittings of the reflectivity and the frequency dependent conductivity. Further, no temperature dependence of the reflectivity above 0.2 eV was observed between 25K and 200K. The present study suggests the inapplicability of the Drude model to this system and supports the existence of the mid-IR absorption.