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The partial (up to 7 %) substitution of Cd for Zn in the Yb-basedheavy-fermion material YbFe$_2$Zn$_{20}$ is known to induce a slight ($\sim 20$%) reduction of the Sommerfeld specific heat coefficient $\gamma$ and a huge(up to two orders of magnitude) reduction of the $T^2$ resistivity coefficient$A$, corresponding to a drastic and unexpected reduction of the Kadowaki-Woodsratio $A/\gamma ^2$. Here, Yb $L_{3}$-edge X-ray absorption spectroscopy showsthat the Yb valence state is close to $3+$ for all $x$, whereas X-raydiffraction reveals that Cd replace the Zn ions only at the $16c$ site of the$Fd\bar{3}m$ cubic structure, leaving the $48f$ and $96g$ sites with full Znoccupation. Ab-initio electronic structure calculations in pure and Cd-dopedmaterials, carried out without considering correlations, show multipleconduction bands with only minor modifications of the band dispersions near theFermi level and therefore do not explain the resistivity drop introduced by Cdsubstitution. We propose that the site-selective Cd substitution introduceslight conduction bands with substantial contribution of Cd($16c$) $5p$ levelsthat have weak coupling to the Yb$^{3+}$ $4f$ moments. These light fermionscoexist with heavy fermions originated from other conduction bands with largerparticipation of Zn($48f$ and $96g$) $4p$ levels that remain strongly coupledwith the Yb$^{3+}$ local moments.

physical review. b./physical review. b

Crystal, local atomic and electronic structures of  YbFe$_2$Zn$_{20-x}$Cd$_x$ ($0 \leq x \leq 1.4$): a multi-band system with  possible coexistence of light and heavy fermions