Zooming-in on the charge ordering in YBa2Cu3O6.5

We report direct evidence of charge/orbital ordering of low energy electronic states of $Cu$ in YBa${}_2$Cu${}_3$O${}_{6+x}$ ortho-II phase in both the $Cu{O}_3$ chain and the CuO${}_2$ plane. Huge enhancement of the $(1\mathrm{/}2,0,0)$ superstructure Bragg peak is observed when photon energy is tuned to the $Cu L_{2,3}$ absorption edge with large polarization dependence. The ordering in the $Cu{O}_2$ plane discovered here sheds new light on how the one dimensional $Cu-O$ chains affect the $Cu{O}_2$ plane, and why many normal and superconducting state properties of this system exhibit strong anisotropy.