Optical conductivity [Ο(Ο)] of the Kondo semiconductor YbB12β
has been measured over wide ranges of temperature (T=8β690 K) and photon
energy (βΟβ₯ 1.3 meV). The Ο(Ο) data reveal the
entire crossover of YbB12β from a metallic electronic structure at high T
into a semiconducting one at low T. Associated with the gap development in
Ο(Ο), a clear onset is newly found at βΟ=15 meV for Tβ€ 20 K. The onset energy is identified as the gap width of YbB12β
appearing in Ο(Ο). This gap in \sigma(\omega)isinterpretedastheindirectgap,whichhasbeenpredictedinthebandmodelofKondosemiconductor.Ontheotherhand,thestrongmidβinfrared(mIR)peakobservedin\sigma(\omega)$ is interpreted as arising from the direct gap. The
absorption coefficient around the onset and the mIR peak indeed show
characteristic energy dependences expected for indirect and direct optical
transitions in conventional semiconductors.Comment: 4 pages, 3 figures, submitted to J. Phys. Soc. Jp