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.

The optical conductivity [ð!Þ] of the Kondo semiconductor YbB 12 has been measured over wide ranges of temperature (T ¼ 8{690 K) and photon energy (h ! ! 1:3 meV). The ð!Þ data reveal the entire crossover of YbB 12 from a metallic electronic structure at high T's to a semiconducting one at low T's. Associated with the gap development in ð!Þ, a clear onset is newly found at h ! ¼ 15 meV for T 20 K. The onset energy is identified as the gap magnitude of YbB 12 appearing in ð!Þ. This gap in ð!Þ is interpreted as the indirect gap, which has been predicted by the renormalized-band model of the Kondo semiconductor. On the other hand, the strong mid-infrared (mIR) peak observed in ð!Þ is interpreted as arising from the direct gap. The absorption coefficient around the onset and the mIR peak indeed show the characteristic energy dependences expected for indirect and direct optical transitions in conventional semiconductors.

Optical reflectivity [R(ω)] of YbInCu4 single crystals has been measured across its first order valence transition at Tv ≃ 42 K, using both polished and cleaved surfaces. R(ω) measured on cleaved surfaces [Rc(ω)] was found much lower than that on polished surface [Rp(ω)] over the entire infrared region. Upon cooling through Tv, Rc(ω) showed rapid changes over a temperature range of less than 2 K, and showed only minor changes with further cooling. In contrast, Rp(ω) showed much more gradual and continuous changes across Tv, similarly to previously reported data on polished surfaces. The present result on cleaved surfaces demonstrates that the microscopic electronic structures of YbInCu4 indeed undergo sudden changes upon the valence transition, which is consistent with its first order nature. The gradual temperature evolution of Rp(ω) is most likely due to the compositional and/or Yb-In site disorders caused by polishing.

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