Optical conductivity of the Kondo insulator YbB_12: Gap formation and low-energy excitations

TL;DR

This study investigates the optical conductivity of YbB_12, revealing a gap formation below 70 K and identifying key low-energy excitations linked to Yb 4f states and band transitions.

Contribution

First detailed optical conductivity measurements of YbB_12 showing gap formation and associated electronic transitions in a Kondo insulator.

Findings

Energy gap of ~25 meV opens below 70 K

Narrow peak at ~40 meV linked to Yb 4f transitions

Broad peak at ~0.25 eV related to Yb 4f and p-d band transitions

Abstract

Optical reflectivity experiments have been conducted on single crystals of the Kondo insulator YbB_12 in order to obtain its optical conductivity, \sigma(\omega). Upon cooling below 70 K, a strong supression of \sigma(\omega) is seen in the far-infrared region, indicating the opening of an energy gap of ~ 25 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 ~40 meV in \sigma(\omega), which is attributed to optical transitions between the Yb 4f-derived states across the gap. In addition, a broad peak is observed at ~0.25 eV. This peak is attributed to transitions between Yb 4f-derived states and p-d band, and is reminiscent of similar peaks previously observed for rare-earth hexaborides.