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, SmB$_6$ and YbB$_{12}$, 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 Sm$^{3+}$ ions in SmB$_6$ were detected, and the formation of Yb$^{3+}$ ion pairs and the spontaneous breaking of cubic symmetry in YbB$_{12}$ 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.