Evidence of charge stripes, charge-spin-orbital coupling and phase transition in a simple copper oxide CuO

There is a great current interest in understanding the role of charge stripes in high-T_c superconductivity. We found that static charge ordering and alignment of charge-ordered domains analogous to the charge stripes even exist in a compound as simple as cupric oxide, CuO. The behavior of the holes (charges) in charge-ordered CuO was extensively investigated by transmission electron microscopy, X-ray diffraction, charge transport, dielectric constant, magnetic susceptibility, and heat capacity measurements using clean single crystal samples. Correlation of charge, spin and lattice was clarified by these measurements. Further, we found a high temperature phase transition around 880K below which charge ordering, enhanced lattice distortion and a possible orbital coupling due to the exchange of d_x2-y2 orbital of Cu, were shown to be developed. The present discovery suggests that charge stripe is quite possibly an inherent property of copper oxides and that CuO is a reference system for studying charge-spin-orbital correlation.