OPTICAL CONDUCTIVITY OF THE KONDO INSULATOR YBB12 - GAP FORMATION ANDLOW-ENERGY EXCITATIONS

Optical reflectivity experiments have been conducted on single crystal s of the Kondo insulator YbB12 in order to obtain its optical conducti vity, sigma(omega). Upon cooling below 70 K, a strong suppression of s igma(omega) is seen in the far-infrared region, indicating the opening of an energy gap of similar to 25 meV. This gap development is coinci dent 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 similar to 40 meV in sigma(o mega), which is attributed to optical transitions between the Yb 4f-de rived states across the gap. In addition, a broad peak is observed at similar to 0.25 eV. This peak is attributed to transitions between Yb 4f-derived states and the p-d band, and is reminiscent of similar peak s previously observed for rare-earth hexaborides. [S0163-1829(98)52236 -9].