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

Abstract

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 Ru4+ state (4d4) is usually stable in these materials. In this regard, Ru3+ ions in perovskite materials are expected to be a mesmerizing 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 Ru4+ ions, e.g., SrRuO3. We therefore argue that the core level spectra of LaRuO3 and NdRuO3 are peculiar to the Ru3+ states.