Localized magnetic excitation in the hybridization gap of YbAl$_{3}$

YbAl$_{3}$ is an intermediate valence (IV) compound which enters the coherent Fermi liquid phase below T$_{coh}$ = 50K.$^{[1]}$ We have recently measured the magnetic scattering on the MAPS spectrometer at ISIS using high-quality single crystals. For T $<$ 50K, the data can be fit as the sum of a background of nonmagnetic scattering and a pair of peaks at E$_{1}$ = 50meV and E$_{2}$ = 33meV which, in the extended zone scheme, scale with Q as the 4f form factor, as expected for magnetic scattering. The scattering near 50meV exhibits a peak in intensity near Q = (1.2, 0.5, 0.5) which also disperses somewhat with Q. Such Q-dependence is as expected for interband scattering across the hybridization gap in IV compounds. The scattering near 33meV, however, is independent of Q in both intensity and position and hence is the result of a spatially localized excitation. The energy of this excitation coincides with a deep minimum in the optical conductivity$^{[2]}$, and hence the excitation energy lies in the middle of the hybridization gap. Both the magnetic excitation and the deep minimum in the conductivity gradually disappear above 50K, indicating that they are properties of the renormalized ground state. 1 A. L. Cornelius, et al, Phys. Rev. Lett. 88 (2002) 117201. 2. H. Okamura et al, Journ. Phys. Soc. Japan 73 (2004) 2045.