Optical Investigation of a Nonsuperconducting Nd1.85Ce0.15CuO4+ y Crystal with Possible Charge Ordering Instability.

We have investigated the temperature variation of optical spectra comparatively for an oxygenated nonsuperconducting and a reduced superconducting crystal of Nd_1.85 Ce_0.15 CuO_4+y . Anomalous gap-like structure as large as 0.3 eV has been observed in the optical conductivity spectrum of the oxygenated crystal with lowering temperature from 340 K, while little temperature-variation has been observed for the reduced crystal. Furthermore, anomalies in lattice dynamics have been observed only for the oxygenated crystal in far-infrared spectra. These spectral signatures have shown parallel temperature-dependence, signaling a possible charge ordering in the oxygenated nonsuperconducting crystal. [ABSTRACT FROM AUTHOR]