Electronic structure ofLa1−xSrxMnO3studied by photoemission and x-ray-absorption spectroscopy

Abstract

The electronic structure of ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{MnO}}_{3}$ has been studied by photoemission and O 1s x-ray-absorption spectroscopy. Spectra of the Mn 2p core levels and the valence bands for ${\mathrm{LaMnO}}_{3}$ and ${\mathrm{SrMnO}}_{3}$ have been analyzed using a configuration-interaction cluster model. The ground state of ${\mathrm{LaMnO}}_{3}$ is found to be mixed ${\mathit{d}}^{4}$ and ${\mathit{d}}^{5}$L states and that of ${\mathrm{SrMnO}}_{3}$ to be heavily mixed ${\mathit{d}}^{3}$ and ${\mathit{d}}^{4}$L states, reflecting their strong covalency. The character of the band gap of ${\mathrm{LaMnO}}_{3}$ is of the p-to-d charge-transfer type while that of ${\mathrm{SrMnO}}_{3}$ has considerable p-p character as well as p-d character. Holes doped into ${\mathrm{LaMnO}}_{3}$ mainly of oxygen p character are coupled antiferromagnetically with the ${\mathit{d}}^{4}$ local moments of the ${\mathrm{Mn}}^{3+}$ ions and become itinerant, thus aligning the Mn moments ferromagnetically. The changes in the electronic structure with carrier doping are not of the rigid band type: By La substitution for ${\mathrm{SrMnO}}_{3}$, the so-called in-gap spectral weight (of ${\mathit{e}}_{\mathit{g}\mathrm{\ensuremath{\uparrow}}}$ symmetry) appears with its peak located 1--2 eV below the Fermi level and grows in intensity with increasing La concentration, while the spectral intensity of the ${\mathit{e}}_{\mathit{g}\mathrm{\ensuremath{\uparrow}}}$ states above the Fermi level decreases, showing a transfer of spectral weight from the unoccupied to the occupied ${\mathit{e}}_{\mathit{g}\mathrm{\ensuremath{\uparrow}}}$ states with electron doping. Meanwhile, the intensity at the Fermi level remains low even in the metallic phase (0.2\ensuremath{\lesssim}x\ensuremath{\lesssim}0.6). The energy shifts of core-level peaks and valence-band features with x suggest a downward shift of the Fermi level with hole doping, but the shift is found to be very small in the metallic phase. The importance of the orbital degeneracy of the ${\mathit{e}}_{\mathit{g}\mathrm{\ensuremath{\uparrow}}}$ band and possible orbital fluctuations in the ferromagnetic phase are pointed out.