Photoemission study ofYb2Co3X9(X=Ga,Al):Variation of the electronic structure from a mixed-valent to Kondo-lattice system

We have studied the electronic structures of ${\mathrm{Yb}}_{2}{\mathrm{Co}}_{3}{X}_{9}$ $(X=\mathrm{Ga}$ and Al) compounds by high-resolution photoemission spectroscopy. The isostructural series of compounds ${\mathrm{Yb}}_{2}{T}_{3}{X}_{9}$ $(T=\mathrm{Co},\mathrm{Rh},\mathrm{Ir};$ $X=\mathrm{Ga},\mathrm{Al})$ are known to be antiferromagnetic Kondo lattice for $X=\mathrm{Al},$ while they show mixed-valent behavior for $X=\mathrm{Ga}.$ In the measurements at 14 K, we observed a sharp divalent Yb $4f$ peak just below ${E}_{F}$ for ${\mathrm{Yb}}_{2}{\mathrm{Co}}_{3}{\mathrm{Ga}}_{9}$ while no $4f$-related signal could be clearly identified just below ${E}_{F}$ for $X=\mathrm{Al}.$ Both the position and the width of the peak observed for $X=\mathrm{Ga}$ are comparable to ${k}_{B}{T}_{K},$ where ${T}_{K}$ is the Kondo temperature, and the intensity of the peak shows a strong temperature dependence. The result is generally consistent with the behavior of the Kondo peak predicted by the single-impurity Anderson model. The origin of the difference between the mixed-valent Ga-based compounds and the Kondo-lattice Al-based compounds is attributed both to the difference in the $4f$ energy position and the $4f$-ligand hybridization strength. The line shape of the Kondo peak on the higher binding-energy side is, however, significantly different from that predicted by the calculation, which we attribute to the excitation of low-energy electron-hole pairs across the Fermi level neglected in the noncrossing approximation treatment of the single-impurity Anderson model.