Out-of-plane orbital characters of intrinsic and doped holes in La_2-xSr_xCuO₄

Bulk-sensitive, polarization-dependent oxygen K- and copper L-edge absorption spectra of single-domain La_2-xSr_xCuO₄ with CuO₂ 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 O2p_z character, likely associated with apical O, but a very weak Cu3d_3z²-r² character. The quantitative results impose strong contraints on superconductivity models invoking out-of-plane orbitals, and suggest that the apical O 2p_z orbital may play an important role in high-T_c phenomena.