Electronic structure and f-orbital occupancy in Yb-substituted CeCoIn5

The local structure and 4f orbital occupancy have been investigated in Ce1-x YbxCoIn5 via Yb L-III-edge extended x-ray absorption fine structure (EXAFS), Ce and Yb L-III-edge x-ray absorption near-edge structure (XANES), and angle-resolved photoemission spectroscopy (ARPES) measurements. Yb(III) (4f(13)) is the hole analog of Ce(III) (4f(1)). Yb is found to be strongly intermediate-valent in Ce1-x YbxCoIn5 throughout the entire doping range, including pure YbCoIn5, with an f-hole occupancy for Yb of n(f) similar or equal to 0.3 (i.e., Yb2.3+), independent of Yb concentration and independent of temperature down to T = 20 K. In contrast, the f-electron orbital occupancy for Ce remains close to 1 for all Yb concentrations, suggesting that there is no mutual influence on n(f) between neighboring Ce and Yb sites. Likewise, ARPES measurements at 12 K have found that the electronic structure along Gamma - X is not sensitive to the Yb substitution, suggesting that the Kondo hybridization of Ce f electrons with the conduction band is not affected by the presence of Yb impurities in the lattice. The emerging picture is that in Ce1-x YbxCoIn5 there are two networks, interlaced but independent, that couple to the conduction band: one network of Ce ions in the heavy-fermion limit, and one network of Yb ions in the strongly intermediate-valent limit. The robustness of the local and electronic structure to doping suggests the absence of charge transfer between the Ce and Yb ions, and may explain the relative robustness of superconductivity for this Ce-site substitution as compared to the In-site substitution.