Yb valence change in Ce1-xYbxCoIn5 from spectroscopy and bulk properties

The electronic structure of Ce1-xYbxCoIn5 has been studied by a combination of photoemission, x-ray absorption, and bulk property measurements. Previous findings of a Ce valence near 3+ for all x and of an Yb valence near 2.3+ for x >= 0.3 were confirmed. One new result of this study is that the Yb valence for x <= 0.2 increases rapidly with decreasing x from 2.3 toward 3+, which correlates well with de Haas van Alphen results showing a change of Fermi surface around x = 0.2. Another new result is the direct observation by angle resolved photoemission Fermi surface maps of approximate to 50% cross-sectional area reductions of the alpha and beta sheets for x = 1 compared to x = 0, and a smaller, essentially proportionate, size change of the a sheet for x = 0.2. These changes are found to be in good general agreement with expectations from simple electron counting. The implications of these results for the unusual robustness of superconductivity and Kondo coherence with increasing x in this alloy system are discussed.