Electronic structure and Evolution of Orbital State in Metallic Ca$_{2-x}$Sr$_x$RuO$_4$

We investigated electronic structure of a layered Ca$_{2-x}$ Sr$_x$RuO$_4$ in a metallic regime (0.15 $\le$ x $\le$ 2.0) using polarization dependent O 1s x-ray absorption spectroscopy. The spectrum shows strong variation with the polarization especially in Ru 4d region, which enables us to identify the electronic states. The spectral line shape gradually changes with increase of Sr concentration, and agrees well with the unrestricted Hartree-Fock analysis. The analysis suggests evolution of the orbital states and provides clues for how the lattice distortion affects the orbital occupations.