Electronic structures of layered perovskite Sr2MO4 (M=Ru, Rh, and Ir)

We investigated the electronic structures of the two-dimensional layered perovskite Sr${}_2$$\mathit{\text{M}}$O${}_4$ ($\mathit{\text{M}}$=4$\mathit{\text{d}}$ Ru, 4$\mathit{\text{d}}$ Rh, and 5$\mathit{\text{d}}$ Ir) using optical spectroscopy and polarization-dependent O 1$\mathit{\text{s}}$ x-ray absorption spectroscopy. While the ground states of the series of compounds are rather different, their optical conductivity spectra $\sigma (\omega )$ exhibit similar interband transitions, indicative of the common electronic structures of the 4$\mathit{\text{d}}$ and 5$\mathit{\text{d}}$ layered oxides. The energy splittings between the two $e_g$ orbitals, $i\mathrm{.}e\mathrm{.}$, $d_{3z^2-r^2}$ and $d_{x^2-y^2}$, are about 2 eV, which is much larger than those in the pseudocubic and 3$\mathit{\text{d}}$ layered perovskite oxides. The electronic properties of the Sr${}_2$$\mathit{\text{M}}$O${}_4$ compounds are discussed in terms of the crystal structure and the extended character of the 4$\mathit{\text{d}}$ and 5$\mathit{\text{d}}$ orbitals.