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

Abstract

The $d^5$ 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 ($4d^4$) is usually stable in these materials. In this regard, Ru${}^{3+}$ ions in perovskite materials are expected to be a mesmerising playground of $4d^5$ electron configurations. Here, we report measurements of x-ray photoemission spectroscopy on recently synthesized perovskite ruthenium oxide thin films, LaRuO${}_3$ and NdRuO${}_3$, 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 Sr${}_2$RuO${}_4$. 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., SrRuO${}_3$. We therefore argue that the core level spectra of LaRuO${}_3$ and NdRuO${}_3$ are peculiar to the Ru${}^{3+}$ states.

5 pages, 4 figures