Electronic structures of layered perovskiteSr2MO4(M=Ru, Rh, and Ir)

We investigated the electronic structures of the two-dimensional layered perovskite ${\mathrm{Sr}}_{2}M{\mathrm{O}}_4$ ($M=4d$ Ru, $4d$ Rh, and $5d$ Ir) using optical spectroscopy and polarization-dependent O $1s$ x-ray absorption spectroscopy. While the ground states of the series of compounds are rather different, their optical conductivity spectra $\sigma \left(\omega \right)$ exhibit similar interband transitions, indicative of the common electronic structures of the $4d$ and $5d$ layered oxides. The energy splittings between the two $e_g$ orbitals, i.e., $d_{3z^2-r^2}$ and $d_{x^2-y^2}$, are about $2\mathrm{eV}$, which is much larger than those in the pseudocubic and $3d$ layered perovskite oxides. The electronic properties of the ${\mathrm{Sr}}_{2}M{\mathrm{O}}_4$ compounds are discussed in terms of the crystal structure and the extended character of the $4d$ and $5d$ orbitals.