Hard and soft x-ray photoemission spectroscopy study of the new Kondo system SmO thin film

SmO thin film is a new Kondo system showing a resistivity upturn around 10 K and was theoretically proposed to have a topologically nontrivial band structure. We perform hard x-ray and soft x-ray photoemission spectroscopy to elucidate the electronic structure of SmO. From the Sm $3d$ core-level spectra, we estimate the valence of Sm to be $\ensuremath{\sim}2.96$, proving that the Sm has a mixed valence. The valence-band photoemission spectra exhibit a clear Fermi edge originating from the Sm $5d$-derived band. The present finding is consistent with the theory suggesting a possible topological state in SmO and show that rare-earth monoxides or their heterostructures can be a new playground for the interplay of strong electron correlation and spin-orbit coupling.