Neutron scattering study of spin and lattice dynamics in YbB12

The spin dynamics of mixed-valence YbB12 has been studied by inelastic neutron scattering on a single crystal. In the Kondo-insulating regime, the spectra exhibit a spin-gap structure with two dispersive in-gap excitations at omega{approximately equal to}15 and 20meV. The lower mode is associated with short-range correlations near the antiferromagnetic wave vector . Its properties are in overall agreement with those expected for a 'spin exciton' branch in the hybridization-gap picture, but Lu dilution experiments favor a more local mechanism for the gap formation. The vibrational behavior of Yb ions within the rigid B lattice is reflected into details of the phonon dispersion curves.