Evolution of electronic states in the Kondo alloy systemYb1−xLuxB12

We have studied the effect of Lu substitution on the Kondo insulator ${\mathrm{YbB}}_{12}$ by high-resolution photoemission. Comparison of the spectra of ${\mathrm{YbB}}_{12}$, ${\mathrm{Yb}}_{0.5}{\mathrm{Lu}}_{0.5}{\mathrm{B}}_{12}$, and ${\mathrm{LuB}}_{12}$ reveals that the density of states (DOS) of the B $\mathrm{sp}$-derived conduction band near the Fermi level is reduced in ${\mathrm{YbB}}_{12}$ over a rather wide (∼ 40 meV) energy region. Lu substitution (i) recovers the reduced B $\mathrm{sp}$ DOS, (ii) shifts the Yb $4f$-derived Kondo peak towards higher binding energy, and (iii) decreases the Yb valence. These results are consistently analyzed using the Anderson-impurity model, and imply interaction between the Yb $4f$ ions mediated by the Yb $4f$-B $\mathrm{sp}$ hybridization in ${\mathrm{YbB}}_{12}.$