Pseudogap formation and heavy-carrier dynamics in intermediate-valence YbAl3

Infrared optical conductivity [sigma(omega)] of the intermediate-valence compound YbAl3 has been measured at temperatures 8-690 K to study its microscopic electronic structures. Despite the highly metallic characteristics of YbAl3, 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 similar to0.25 eV. The energy-dependent effective mass and scattering rate of the carriers obtained from the data indicate the formation of a heavy-mass Fermi liquid. These characteristic results are discussed in terms of the hybridization states between the Yb 4f and the conduction electrons. It is argued, in particular, that the pseudogap and the mid-infrared peak result from the indirect and direct gaps, respectively, within the hybridization state.