Site-specific and doping-dependent electronic structure of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ probed by O 1s and Cu 2p x-ray-absorption spectroscopy

The electronic structure of the ${\mathrm{CuO}}_2$ planes and ${\mathrm{CuO}}_3$ chains in single-domain crystals of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ has been investigated as a function of oxygen concentration (6≤x≤7) using polarization-dependent x-ray-absorption spectroscopy of the O 1s and Cu 2p core levels. The polarization-dependent observation of unoccupied states with O and with Cu orbital character parallel to the a, b, and c axes of the crystals allows the determination of the number of hole states in Cu 3${\mathit{d}}_{\mathit{x}}^2$-${\mathit{y}}^2$ and O 2${\mathit{p}}_{\mathit{x},}$y orbitals in the ${\mathrm{CuO}}_2$ planes as well as in Cu 3${\mathit{d}}_{\mathit{y}}^2$-${\mathit{z}}^2$ and O 2${\mathit{p}}_{\mathit{y},}$z orbitals in the ${\mathrm{CuO}}_3$ chains. States with Cu 3${\mathit{d}}_{3\mathit{z}}^2$-${\mathit{r}}^2$ orbital character contribute less than 10% to the total number of states near the Fermi level. The number of holes in the planes and in the chains is found to be correlated with the superconducting transition temperature.