Optical conductivity of the Kondo insulator Gap formation and low-energy excitations

Abstract

Optical reflectivity experiments have been conducted on single crystals of the Kondo insulator (Formula presented) in order to obtain its optical conductivity, σ(ω). Upon cooling below 70 K, a strong suppression of σ(ω) is seen in the far-infrared region, indicating the opening of an energy gap of (Formula presented) 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 (Formula presented) in σ(ω), which is attributed to optical transitions between the Yb (Formula presented)-derived states across the gap. In addition, a broad peak is observed at (Formula presented) This peak is attributed to transitions between Yb (Formula presented)-derived states and the (Formula presented) band, and is reminiscent of similar peaks previously observed for rare-earth hexaborides. © 1998 The American Physical Society.