Spin-gap magnetic response in (Yb, Lu)B12

To clarify the role of Yb–Yb correlations in the formation of the gap-like excitation spectrum of YbB12, the spin dynamics of strongly diluted Yb0.25Lu0.75B12 have been studied by inelastic neutron scattering in a wide temperature range. The data indicate that the spin gap is not suppressed by the dilution process even for large concentrations of Lu. However, the breaking-down of the Yb-sublattice periodicity leads to a strong smearing of the low-energy features and to a moderate suppression of the high-energy peak in the magnetic spectral response of YbB12. The interrelation of the spin and charge gaps is discussed.