Photoelectron spectroscopy of strongly correlated Yb compounds

The electronic properties of the Yb compounds YBCu2Si2, YBAgCu4, and YbAl3 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) similar to 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 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.