Unoccupied electronic structure of Sr 2 CuO 2 Cl 2 and Ba 2 Cu 3 O 4 Cl 2 : Experiment and theory

The unoccupied electronic structure of the layered cuprates ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2}$ and ${\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{4}{\mathrm{Cl}}_{2}$ has been studied using polarization-dependent x-ray absorption spectroscopy at the O $1s$ and Cu ${2p}_{3/2}$ edges and band-structure calculations within the local-density approximation. In contrast to almost all high-temperature superconductors, there are no oxygen atoms outside the Cu-O planes in these oxychlorides. Our results represent therefore direct experimental information regarding the oxygen-derived unoccupied electronic structure of undoped Cu-O planes in square-planar coordination. The O 1$s$ and Cu ${2p}_{3/2}$ x-ray absorption spectra of ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2}$ and ${\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{4}{\mathrm{Cl}}_{2}$ are quite similar with an essentially two-dimensional upper Hubbard band and extra oxygen and copper related absorption fine structure at higher energies. From its polarization dependence and a comparison to the results of the band-structure calculations, it is concluded that this extra fine structure is related to transitions into O $2p$ and Cu ${3d}_{{3z}^{2}\ensuremath{-}{r}^{2}}$ orbitals which have become partly unoccupied due to hybridization with Sr $4d/\mathrm{B}\mathrm{a}$ $5d/\mathrm{C}\mathrm{u}$ ${4p}_{z}$ and Cu $4s$ orbitals, respectively. Differences between these two compounds are the larger width of the upper Hubbard band and the higher relative Cu ${3d}_{{3z}^{2}\ensuremath{-}{r}^{2}}$ hole occupation above the upper Hubbard band in ${\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{4}{\mathrm{Cl}}_{2}$. These differences can be related to the extra ${\mathrm{Cu}}_{B}$ atoms in the ${\mathrm{Cu}}_{3}{\mathrm{O}}_{4}$ planes of ${\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{4}{\mathrm{Cl}}_{2}$ with respect to the ${\mathrm{CuO}}_{2}$ planes of ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2}$.