Heavy Quasiparticles and Pseudogap Formation in Ybal 3 : Optical Conductivity Study. *

We have measured the optical conductivity σ(ω) of the mixed-valent compound YbAl3. σ(ω) exhibits a mid-infrared peak centered at 0.150.2 eV, which becomes more pronounced with decreasing temperature (T ). In addition, a strong depletion of the spectral weight, i.e., a pseudogap formation, is observed in σ(ω) below ∼ 0.1 eV. A comparison of σ(ω) with the dc conductivity indicates the existence of an extremely narrow Drude peak at very low energy. Energy-dependent effective mass and scattering rate of the carriers are evaluated from the optical data, which indicates the formation of a heavy-mass Fermi liquid state within ∼ 40 meV from the Fermi level. These observations are discussed in terms of the hybridization of a conduction band and a narrow 4f band.