Observation of Charge Stripes in Cupric Oxide

One of the present intensive concerns about the high-temperature superconductors is whether charge stripes are a key to superconductivity. Here we report observation of charge stripes in the simplest copper oxide, CuO, by real-space images obtained by electron microscopy. Charge-ordered domains and normal-lattice domains exist alternatively in the vapor-grown single crystal of CuO. Since CuO consists of the Cu-O bonding, which is a basic material feature for high- $T_c$ cuprates, the discovery of charge stripes in this basic compound has important implications for discussing the mechanism of superconductivity in complex cuprates.