Pseudogap Formation and Heavy Carrier Dynamics in Intermediate Valence YbAl3

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Infrared optical conductivity [σ(ω)] of the intermediate valence compound YbAl₃ has been measured at temperatures 8 K ≤ T ≤ 690 K to study its microscopic electronic structures. Despite the highly metallic characters of YbAl₃, σ(ω) 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 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 4f 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.