Surface core-level shifts and electronic structures of Yb compounds studied with use of photoemission spectroscopy

Mixed-valent Yb compounds (${\mathrm{YbIn}}_2$, ${\mathrm{Yb}}_4$ ${\mathrm{As}}_3$, ${\mathrm{YbCu}}_2$, ${\mathrm{YbAl}}_3$, ${\mathrm{Yb}}_4$ ${\mathrm{Sb}}_3$, ${\mathrm{Yb}}_4$ ${\mathrm{Bi}}_3$) and divalent ${\mathrm{YbPb}}_3$ were studied by high-resolution photoemission spectroscopy with the use of synchrotron radiation in the photon energy range 40≤ħω≤140 eV. It was found that the divalent 4f spectra in all of these compounds consist of two components, one from the bulk and the other from the surface layer(s). The surface components are shifted to higher binding energies by 0.52–0.94 eV. These surface core-level shifts are reasonably well described by the Johansson and Mårtensson fully screened core-hole model with Miedema’s empirical scheme for cohesive energy estimation. The 4d or 5d core levels of some anion atoms are also observed to be shifted at the surface to the higher binding energies by 0.36–0.56 eV, suggesting that the charge transfer between cation and anion atoms is not the origin of surface core-level shifts in these compounds. The electron mean free path is found to decrease with decreasing kinetic energies of the photoelectron down to ħω=40 eV in these compounds. The bulk Yb valence v and the 4f level binding energy ${\mathrm{\varepsilon }}_{\mathit{f}}$ for each compound are also deduced from the bulk 4f spectral features. The zero-temperature magnetic susceptibility χ(0) estimated using the lowest-order analytic relation for the Anderson impurity Hamiltonian from these values is in good agreement with available experimental data, suggesting that the Anderson impurity Hamiltonian description is a good starting point for the electronic structures of these Yb compounds. The hybridization strength Δ between the 4f level and conduction electrons is found to be in the range of 17–78 meV.