Spectroscopic investigations on trivalent ruthenium ions in ruthenium perovskite oxide thin films

The d⁵ electron configurations under the crystal field, spin-orbit coupling, and Coulomb interaction give rise to a plethora of profound ground states. Ruthenium perovskite oxides exhibit a number of unconventional properties yet the Ru^4+ state (4d⁴) is usually stable in these materials. In this regard, Ru^3+ ions in perovskite materials are expected to be a mesmerising playground of 4d⁵ electron configurations. Here, we report measurements of x-ray photoemission spectroscopy on recently synthesized perovskite ruthenium oxide thin films, LaRuO₃ and NdRuO₃, whose valence state of the ruthenium ions is trivalent. We discuss correlation and spin-orbit effects from the valence-band spectra, in particular an additional peak structure around 3-5 eV, reminiscent of the so-called 3 eV peak observed in Sr₂RuO₄. Moreover, we find that the core-level spectra of these materials are quantitatively different from those in other ruthenates which possess Ru^4+ ions, e. g. , SrRuO₃. We therefore argue that the core level spectra of LaRuO₃ and NdRuO₃ are peculiar to the Ru^3+ states.