Photoelectron spectroscopy of strongly correlated Yb compounds

The electronic properties of the Yb compounds YB${\mathrm{Cu}}_2$ ${\mathrm{Si}}_2$, YBAg${\mathrm{Cu}}_4$, and Yb${\mathrm{Al}}_3$ along with purely divalent Yb metal, have been investigated by means of high-resolution ultraviolet and x-ray photoelectron spectroscopy. We present the intrinsic characteristic features of the $4f$ levels of Yb while accounting for lattice vibrations and the manifestation of corelike energy levels degenerate with the valence states and modified by the temperature-dependent Fermi function. For these strongly correlated Yb-based compounds, the hole occupancy values ($n_f\sim 0.6$) directly obtained from integration of the divalent and trivalent portions of the $4f$ photoemission features indicate that these compounds are strongly mixed valent. The small intensity modulation with temperature in the divalent Yb $4f$ levels (0-10% over a $T=20-300\mathrm{}$ K range) is discussed within the conventional framework of the photoemission process and nominal allowances for lattice variations with temperature. Results from photoemission experiments on the divalent $4f$ levels of strongly correlated Yb compounds are remarkably similar to the $4f$ levels of purely divalent Yb metal.