Observation off-Band Hybridization Gap in the Anomalous Rare-Earth Compound YbAl3

Low-temperature (< 10 K) paramagnetic spectral response ${\chi }^{\prime \prime }(Q, \omega )$ in Yb${\mathrm{Al}}_3$ shows an abrupt step around ∼ 30 meV extending to ∼ 100 meV. The forward edge of the step broadens markedly and moves to lower energies with a relatively small increase in temperature (to 30 K). Disordering the Yb sublattice by dilution with Lu wipes out the inelastic step, replacing it by a broad distribution. The results give evidence of excitations across a hybridization gap in the $f$ band.