Pseudogap Formation and Heavy Carrier Dynamics in Intermediate Valence YbAl3

Infrared optical conductivity [$\sigma(\omega)$] of the intermediate valence compound YbAl$_3$ has been measured at temperatures 8 K $\leq T \leq$ 690 K to study its microscopic electronic structures. Despite the highly metallic characters of YbAl$_3$, $\sigma(\omega)$ exhibits a clear pseudogap (strong depletion of spectral weight) of about 60 meV below 40 K. It also shows a strong mid-infrared peak centered at $\sim$ 0.25 eV. Energy-dependent effective mass and scattering rate of the carriers obtained from the data indicate the formation of a heavy-mass Fermi liquid state. These characteristic results are discussed in terms of the hybridization states between the Yb 4$f$ and the conduction electrons. It is argued, in particular, that the pseudogap and the mid-infrared peak result from the indirect and the direct gaps, respectively, within the hybridization state. band.