Temperature dependence of the Yb 4f spectra in Yb⁴Bi³ probed by photoemission spectroscopy

We have performed a photoemission study of Yb₄Bi₃. The valence band spectra have shown no multiplet structures associated with the trivalent Yb and revealed that the mean valence of the Yb ion is very close to 2.0. The divalent Yb 4f spectra have shown strong surface components besides weak but sharp bulk components. The surface 4f level shift is about 0.45 eV (300K), 0.48 eV (150K) and 0.54 eV (20K). A clear energy shift of the bulk Yb 4f peaks toward EF with decreasing temperature has been observed, although Yb₄Bi₃ shows neither phase transition nor crossover. Such an unusual peak shift of the bulk component may originate from a change of the lattice constant with decreasing temperature. The observed narrowing of the bulk 4f peaks is thought to be due to a phonon broadening (∼7k_BT).