Crystal, local atomic, and electronic structures if YbFe2Zn20-xCdx (0 ≤ x ≤ 1.4): A multiband system with possible coexistence of light and heavy fermions

The partial (up 7%) substitution of Cd for Zn in the Yb-based heavy-fermion material YbFe2Zn20 is known to induce a slight (~20%) reduction of the Sommerfeld specific-heat coefficient γ and a huge (up to two orders of magnitude) reduction of the T2 resistivity coefficient A, corresponding to a drastic and unexpected reduction of the Kadowaki-Woods ratio A⁄γ^2 . Here, Yb L3-edge x-ray absorption spectroscopy shows that the Yb valence state is close to 3+ for all x, whereas x-ray diffraction reveals that Cd replaces the Zn ions only at the 16c site of the Fd3 ̅m cubic structure, leaving the 48f and 96g sites with full Zn occupation. Ab initio electronic structure calculations in pure and Cd-doped materials, carried out without considering correlations, show multiple conduction bands with only minor modifications of the band dispersion near the Fermi level and therefore do not explain the resistivity drop introduced by Cd substitution. We propose that the site-selective Cd substitution introduces light conduction bands with a substantial contribution of Cd (16c) 5p levels that have weak coupling to the Yb3+ 4f moments. These light fermions coexist with heavy fermions that originate from other conduction bands with larger participation of Zn(96g) 4p levels that remain strongly coupled with the Yb3+ local moments.