Localized excitation in the hybridization gap in YbAl3

The intermediate valence compound YbAl3 exhibits a broad magnetic excitation in the inelastic neutron scattering spectrum with characteristic energy E-1 approximate to 50 meV, equal to the Kondo energy (T-K similar to 600-700 K). In the low temperature (T < T-coh similar to 40 K) Fermi liquid state, however, a new peak in the scattering occurs at E-2 approximate to 33 meV, which lies in the hybridization gap that exists in this compound. We report inelastic neutron scattering results for a single-crystal sample. The scattering at energies near E-1 qualitatively has the momentum (Q) dependence expected for interband scattering across the indirect gap. The scattering near E-2 has a very different Q dependence: it is a weak function of Q over a large fraction of the Brillouin zone and is smallest near (1/2,1/2,1/2). A possibility is that the peak at E-2 arises from a spatially localized excitation in the hybridization gap.