Optical study of interactions in a d-electron Kondo lattice with   ferromagnetism
  We report on a comprehensive optical, transport and thermodynamic study of the Zintl compound Yb$_{14}$MnSb$_{11}$, demonstrating that it is the first ferromagnetic Kondo lattice compound in the underscreened limit. We propose a scenerio whereby the combination of Kondo and Jahn-Teller effects provides a consistent explanation of both transport and optical data. 
K. S. Burch, A. Schafgans, N. P. Butch, T. A. Sayles, M. B. Maple, B.   C. Sales, D. Mandrus, and D. N. Basov