We report a comprehensive infrared magnetospectroscopy study of a ${\mathrm{CeRu}}_4{\mathrm{Sb}}_{12}$ compound revealing quasiparticles with a 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 $m^{*}\simeq 70m_b$ ($m_b$ 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.

• Received 30 May 2004

DOI:https://doi.org/10.1103/PhysRevLett.96.017403