Out-of-plane orbital characters of intrinsic and doped holes in ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$

Bulk-sensitive, polarization-dependent oxygen K- and copper L-edge absorption spectra of single-domain ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ with ${\mathrm{CuO}}_2$ planes tilted off the sample surface were measured. A novel experimental configuration was utilized to achieve complete polarization geometry and eliminate optical path variations. Contrary to previous measurements, our data show for both intrinsic and doped holes a significant amount of O2${\mathit{p}}_{\mathit{z}}$ character, likely associated with apical O, but a very weak Cu3${\mathit{d}}_{3\mathit{z}}^2$-${\mathit{r}}^2$ character. The quantitative results impose strong contraints on superconductivity models invoking out-of-plane orbitals, and suggest that the apical O 2${\mathit{p}}_{\mathit{z}}$ orbital may play an important role in high-${\mathit{T}}_{\mathit{c}}$ phenomena.