Optical reflectivity experiments have been made on single crystals of the K
ondo insulator YbB12 at various temperatures. The obtained optical conducti
vity spectra reveal that an energy gap of similar to 25 meV develops upon c
ooling below 70 K. Detailed temperature-evolution of the gap is presented.
In addition, two prominent low-energy excitation peaks are observed, which
are attributed to optical transitions involving the Yb 4f-derived states ne
ar the Fermi level. (C) 1999 Elsevier Science B.V. All rights reserved.