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 ( 4 d 4 ) is usually stable in these materials. In this regard, Ru3+ ions in perovskite materials are expected to be a mesmerizing playground of 4 d 5 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.