Abstract
The spin dynamics of mixed-valence YbB 12 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 ℏ ω ≈ 15 and 20 meV. The lower mode is associated with short-range correlations near the antiferromagnetic wave vector q 0 = ( 1 2 , 1 2 , 1 2 ) . 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.
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