Surface binding energy shifts, mixed valence and localization of 4f electrons: Ce vs. Sm (invited)

Abstract

Synchrotron radiation and x‐ray photoemission studies of ’’divalent’’ SmS, SmSe and ’’trivalent’’ SmSb as well as Sm metal strongly suggest the existence of a surface induced binding energy shift E^s_B (?0.5 eV to greater E_B) of the 4f⁶ configuration in SmS and SmSe. The divalent surface component of Sm metal has been resolved spectroscopically and has E^s_B (4f⁶,⁶H) ∼0.8 eV relative to the Fermi energy E_F. Resonance enhanced 4f emission has been used to determine E_B for the 4f¹ configuration in the metallic chalcogenides CeS, CeSe and CeTe as well as in the pnictides CeAs and CeN. A resonant two‐peak structure located at E_F and at E_B∼1 eV indicates a 4f surface binding energy shift of this magnitude in this fascinating ’’mixed valent’’ nitride. No shift could be resolved in the other Ce compounds having E_B (4f¹) between 1.8 and 2.6 eV. This is assigned to the more ’’band‐like’’, delocalized behaviour of 4f states in CeN. Constant‐final‐ and initial‐state spectra of CeN differ significantly from those of the other Ce compounds having f‐count near to unity. We take this as a further indication of the mixed valent character of CeN. Resonance enhancement of the cross section of bonding levels (other than 4f states) is also observed, allowing to identify 5d mixing in the valence band. The results of these studies will serve as a novel basis for understanding the exact nature of the 4f state and related anomalous (magnetic) properties of these Ce compounds as well as in Ce itself and its intermetallic compounds.