Indirect and direct energy gaps in kondo semiconductor YbB12

The optical conductivity [sigma(omega)] of the Kondo semiconductor YbB12 has been measured over wide ranges of temperature (T = 8-690 K) and photon energy (h omega) > 1.3 meV). The sigma(omega) data reveal the entire crossover of YbB12 from a metallic electronic structure at high T's to a semiconducting one at low T's. Associated with the gap development in sigma(omega), a clear onset is newly found at ha) = 15 meV for T <= 20 K. The onset energy is identified as the gap magnitude of YbB12 appearing in sigma(omega). This gap in sigma(omega) 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 sigma(omega) 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.