Photoemission study of the electronic structure of the Kondo lattices Yb2Pt6X15 (X =Al, Ga)

The electronic structure of Yb-based Kondo lattices Yb2Pt6X15 (X=Al, Ga) has been investigated by means of photoemission spectroscopy using hard x-ray (hν=5.95 keV) and vacuum ultraviolet (hν=182 eV) synchrotron radiation. The Yb 3d spectra of Yb2Pt6X15 showed both Yb2+- and Yb3+-derived structures, directly indicating valence fluctuation. The Yb valences of Yb2Pt6Al15 were estimated to be 2.89±0.01 at 250 K and ∼2.83 at 20 K, while those of Yb2Pt6Ga15 were ∼2.34 at 300 K with almost no temperature dependence. With changing X ions from Al to Ga, the Pt 5d and Pt 4f peaks were shifted to shallower binding energies, and the Yb3+ 4f peaks were shifted to the deeper binding energies. The X dependences of the Yb valence and the Kondo temperature of Yb2Pt6X15 are discussed based on the Yb3+ 4f hole level relative to the Fermi level EF and Pt-derived density of states at EF.