Far-Infrared Transmission Spectra of SmB₆

Abstract

Far-infrared transmission measurements of a single crystal of SmB 6 have been performed over the range from 25 to 150 cm -1 at 1.5 K. The obtained spectrum shows a gradual increase of transmission below around 100 cm -1 , which was first observed in this experiment, and a steep increase in transmission below 35 cm -1 . From this result, we conclude that the absorption edge arising from the energy gap must be around 100 cm -1 . The absorption between 35 and 100 cm -1 is attributed to impurities or defect sub-levels located in the energy gap. This gives a reasonable explanation to the two edges at 35 and 100 cm -1 which have been seen in many other types of measurements.