Heavy fermion fluid in high magnetic fields: an infrared study of   CeRu$_4$Sb$_{12}$
  We report a comprehensive infrared magneto-spectroscopy study of CeRu$_4$Sb$_{12}$ compound revealing quasiparticles with heavy effective mass m$^*$, with a detailed analysis of optical constants in fields up to 17 T. We find that the applied magnetic field strongly affects the low energy excitations in the system. In particular, the magnitude of m$^*$ $\simeq$ 70 m$_b$ (m$_b$ is the quasiparticle band mass) at 10 K is suppressed by as much as 25 % at 17 T. This effect is in quantitative agreement with the mean-field solution of the periodic Anderson model augmented with a Zeeman term. 
S.V. Dordevic, K.S.D. Beach, N. Takeda, Y.J. Wang, M.B. Maple and D.N.   Basov