Yb-Yb correlations and crystal field in the Kondo-insulator YbB 12

The influence of the coherence breakdown in the Yb sublattice on the excitation spectra of YbB12-based systems and the characteristics of the crystal field potential have been studied by inelastic neutron scattering on Yb0.75Lu0.25B12 and Yb0.9Er0.1B12 samples. The crystal field potential has been determined by use of the Er3+ ions as a sensor. The spectral shape of the Yb magnetic response is considerably changed in the near-gap region due to Lu substitution with respect to the pure YbB12. The possible splitting of the Yb3+ ground-state multiplet due to the crystal field appears to be comparable with the gap energy as well as with the Kondo temperature and energy of the only inelastic peak in the YbB12 spectra at T>TKondo.