Surface valence transition in trivalent Eu insulating compounds observed by photoelectron spectroscopy

The surface valence transition was observed in the insulating rare-earth compounds which are integral valent in the bulk. From the careful study of Eu $3d,$ Eu $4d$, and Eu $4f$ core-level photoelectron spectra of EuF₃, Eu₂O₃, and EuCl₃, we unambiguously identified the divalent layer(s) at the surfaces of these stable trivalent insulating compounds. This finding removes ambiguity and long-standing controversy on the interpretation of core-level satellites in these Eu compounds.