Electron paramagnetic resonance in Kondo insulators

This review is devoted to the application of electron paramagnetic resonance (EPR) in the study of fluctuating-valence materials, which are characterized by a narrow gap in the electron energy spectrum (Kondo insulators or Kondo semiconductors). The authors’ papers on studying classical objects of this field of solid-state physics, SmB6 and YbB12, are considered as an illustration of the potentiality of the EPR method. Temperature dependences of the gap width in these materials were obtained, the static and dynamic Jahn-Teller effects on Sm3+ ions in SmB6 were detected, and the formation of Yb3+ ion pairs and the spontaneous breaking of cubic symmetry in YbB12 were observed. The results obtained indicate that preference should be given to the exciton-polaron model developed by Kikoin et al. for the ground state of Kondo insulators.