Doped holes and Mn valence in manganites: A polarized soft x-ray absorption study of LaMnO₃ and quasi-2D manganite systems

The question of Mn valence and symmetry and population of doped holes in La_1.2Sr_1.65Ca_0.15Mn₂O₇ and LaSr₂Mn₂O₇ bilayer single crystals has been studied with polarized soft x-ray absorption spectroscopy. The observed changes in the O K and Mn L spectra with polarization provide a strong indication for the existence of a competition between the charge dynamics and the lattice distortion that leads to transfer of some of the holes doped in the out-of-plane (3z²-r²) states to the in-plane (x²-y ²) states. The changes observed in these with doping are shown not due to a decrease in the electron population in the d_x ²-y² states but caused by an increase in density of holes in the d_z²-x² and d_z ²-y² states, and the electrons predominantly occupy the corresponding orthogonal states, i.e., d_3x²-r² and d_3y²-r². No evidence is found for the existence of a formal Mn⁴⁺ valence state in these systems.