Indirect and direct energy gaps in the Kondo semiconductor YbB12

TL;DR

This study measures the optical conductivity of YbB12 across a wide temperature and energy range, revealing the evolution from metallic to semiconducting behavior and identifying both indirect and direct energy gaps.

Contribution

It provides experimental evidence for the indirect and direct gaps in YbB12, supporting theoretical band models of Kondo semiconductors.

Findings

Identification of a 15 meV indirect gap at low temperatures.

Observation of a strong mid-infrared peak from the direct gap.

Confirmation of energy dependences consistent with semiconductor optical transitions.

Abstract

Optical conductivity [$\sigma(\omega)$] of the Kondo semiconductor YbB$_{12}$ has been measured over wide ranges of temperature ($T$=8$-$690 K) and photon energy ($\hbar \omega \geq$ 1.3 meV). The $\sigma(\omega)$ data reveal the entire crossover of YbB$_{12}$ from a metallic electronic structure at high $T$ into a semiconducting one at low $T$. Associated with the gap development in $\sigma(\omega)$, a clear onset is newly found at $\hbar\omega$=15 meV for $T \leq$ 20 K. The onset energy is identified as the gap width of YbB$_{12}$ appearing in $\sigma(\omega)$. This gap in \sigma(\omega)$ is interpreted as the indirect gap, which has been predicted in the band model of Kondo semiconductor. On the other hand, the strong mid-infrared (mIR) peak observed in $\sigma(\omega)$ is interpreted as arising from the direct gap. The absorption coefficient around the onset and the mIR peak indeed show characteristic energy dependences expected for indirect and direct optical transitions in conventional semiconductors.