Site-specific unoccupied electronic structure of one-dimensionalSrCuO2

The unoccupied electronic structure of ${\mathrm{SrCuO}}_{2}$ single crystals has been studied using x-ray-absorption spectroscopy. By choosing the orientation of the single crystals with respect to the polarization vector of the synchrotron radiation, we are able to determine the hole occupation of the different O 2p and Cu 3d orbitals. We show that the intrinsic holes in the one-dimensional copper oxide zigzag chains mainly occupy planar Cu ${3\mathrm{d}}_{{\mathrm{y}}^{2}\mathrm{\ensuremath{-}}{\mathrm{z}}^{2}}$ orbitals, which closely resembles the situation in the undoped parent compounds of the high-temperature superconductors. Additionally, most of the oxygen states that are admixed to the upper Hubbard band are concentrated in the center of the chains most probably as a result of the larger number of nearest neighbors. Our results provide an important check for calculations of one-dimensional correlated Cu-O systems.