Ion pairs and spontaneous break of symmetry in the valence-fluctuating compound YbB12

Electron spin resonance (ESR) from Yb3+ ions in a single crystal of fluctuating valence compound YbB12 was studied. We have observed two main lines symmetrically displaced from a g value of 2.55 and exhibiting modulation with 5% anisotropy, when the magnetic field was rotated in the (110) plane from the cubic axis to the [110] direction. Experimental results are explained by an existence of Yb-Yb pairs which are coupled by isotropic exchange but interact also with the other pairs by dipole and exchange coupling. A strong temperature dependence of the amplitude of the ESR signals is found and interpreted as a result of capture of electrons by Yb3+ ions from electron traps with a binding energy of 18 K. The occurrence of a slight anisotropy in a cubic semiconductor may be the result of a spontaneous break of symmetry specific for the ground state of a Kondo dielectric