Surface mixed valence in Sm and SmB6

Abstract

Surface-sensitive photoelectron measurements reveal bulk-to-surface shifts of the Sm $4f^6$ level which imply inhomogeneous valence mixing on the surface of Sm and Sm${\mathrm{B}}_6$. The surface valence fraction is estimated to be the same for both materials. The measurements take advantage of a large resonant enhancement of $4f$ electron emission due to $4d\to 4f$ photon absorption, and detailed spectra showing this phenomena are presented. It is shown that the $4d$ hole in the $4d\to 4f$ absorption process stabilizes the $4f$ state by ∼4-7 eV. Exposure of Sm films to oxygen is found to eliminate, rather than increase, the emission from the surface $4f^6$ state, showing that the $4f^6$ state does not arise from oxygen contamination. Observed variations in Sm film spectra are described, including the finding in some films of an unexplained photoemission peak 2.4 eV below the Fermi level. Sm${\mathrm{B}}_6$ also displays a broad band of Auger emission when a boron $1s$ core hole is created, and this is ascribed to electrons in the boron $2p$ bonding band. Various trends in $4d$ and $4f$ binding energies for Sm and Sm${\mathrm{B}}_6$ are pointed out and discussed.