Valence instability of the samarium metal surface

Abstract
The stability of the surface of the samarium and thulium metals against a divalent state is investigated within the pair-bonding model. Thereby it is found that a divalent surface is favored in samarium and close to becoming stable in thulium. The position of the surface divalent samarium f6 level relative to the Fermi energy is calculated and compared with experiments. Also the surface core-level shifts for the europium and ytterbium metals are considered. Finally some interesting possible physical consequences of the divalent samarium surface layer are pointed out.