Strong electronic correlations in a new class of Yb-based compounds: YbX${\mathrm{Cu}}_4$ (X=Ag,Au,Pd)

A new series of heavy-electron Yb compounds YbX${\mathrm{Cu}}_4$ with X=Ag, Au, or Pd are reported which have electron effective masses as high as ∼60$m_e$, where $m_e$ is the mass of the free electron. Whereas the compounds with X=Au and Pd order magnetically below 1 K, the compound with X=Ag is nonmagnetic. The magnetic susceptibility and specific heat of ${\mathrm{YbAgCu}}_4$ can be accounted for by the Bethe-ansatz solution of the Coqblin-Schrieffer impurity model without any adjustable parameters.