Abstract
Optical reflectivity experiments have been conducted on single crystals of the Kondo insulator ${\mathrm{YbB}}_{12}$ in order to obtain its optical conductivity, \ensuremath{\sigma}(\ensuremath{\omega}). Upon cooling below 70 K, a strong suppression of \ensuremath{\sigma}(\ensuremath{\omega}) is seen in the far-infrared region, indicating the opening of an energy gap of $\ensuremath{\sim}25\mathrm{meV}.$ This gap development is coincident with a rapid decrease in the magnetic susceptibility, which shows that the gap opening has significant influence on magnetic properties. A narrow, asymmetric peak is observed at $\ensuremath{\sim}40\mathrm{meV}$ in \ensuremath{\sigma}(\ensuremath{\omega}), which is attributed to optical transitions between the Yb $4f$-derived states across the gap. In addition, a broad peak is observed at $\ensuremath{\sim}0.25\mathrm{eV}.$ This peak is attributed to transitions between Yb $4f$-derived states and the $p\ensuremath{-}d$ band, and is reminiscent of similar peaks previously observed for rare-earth hexaborides.
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