Correlated Electron State in Ce1-xYbxCoIn5 Stabilized by Cooperative Valence Fluctuations

X-ray diffraction, electrical resistivity, magnetic susceptibility, and specific heat measurements on Ce1-xYbxCoIn5 (0 = x = 1) reveal that many of the characteristic features of the x = 0 correlated electron state are stable for x = 0.775 and that phase separation occurs for x 0.775. The stability of the correlated electron state is apparently due to cooperative behavior of the Ce and Yb ions, involving their unstable valences. Low-temperature non-Fermi liquid behavior is observed and varies with x, even though there is no readily identifiable quantum critical point. The superconducting critical temperature T-c decreases linearly with x towards 0 K as x - 1, in contrast with other HF superconductors where T-c scales with T-coh.