Anomalous Pseudogap Formation in a Nonsuperconducting Crystal ofNd1.85Ce0.15CuO4+y: Implication of Charge Ordering

Temperature variation of optical conductivity and Raman spectra has been investigated comparatively for oxygenated antiferromagnetic and reduced superconducting crystals of ${\mathrm{Nd}}_{1.85}{\mathrm{Ce}}_{0.15}{\mathrm{CuO}}_{4+y}$. While the spectra of the reduced crystal change little with temperature, in the spectra of oxygenated crystal a conspicuous pseudogap structure evolves around 0.3 eV and activated infrared and Raman Cu-O phonon modes grow in intensity, with decreasing temperature from 340 K. The origin is proposed to be charge ordering instability induced by a minute amount of interstitial apical oxygen, which seems to be also responsible for the absence of superconductivity in an oxygenated (or as-grown) crystal.