Spin-charge ordering and charge dynamics in electron-doped high-Tc superconductors

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We have grown single crystals of Nd_<2-x>Ce_xCuO_<4+y> (x=0, 0.05, 0.1, 0.125, 0.15) by traveling solvent floating zone method and have investigated the temperature dependence of charge and lattice dynamics by the measurements of optical reflectivity spectra. In the as-grown crystals with x=0.05, 0.10, 0.125, we have observed pseudo-gap structure and anomalies in phonon modes at low temperatures which are similar to those observed in non-superconducting sample with x=0.15. This indicates the presence of instability to-ward charge-ordering or electronic phase separation for a broad range of doping levels in this system. The difference from the sample with x=0.15 consists of two points, namely, the increased energy of the pseudo-gap structure and the increased onset temperature of the pseudo-gap with reduced doping-levels.We have also investigated the reduced crystal with x=0.125 that is in the vicinity of superconducting-nonsuperconducting trasition point. The resistivity shows metallic be-havior, but at low temperature it shows a modest uptern and does not exhibit the super-conductivity. The pseudo-gap structure has been observed to open up at a slightly higher temperature than the resistivity uptern also in this sample. This strongly suggests that the instability that is responsible for the pseudo-gap suppresses the superconductivity in this electron-doped system.