Robust hybridization gap in a Kondo Insulator YbB${}_{12}$ probed by femtosecond optical spectroscopy

TL;DR

This study uses femtosecond optical spectroscopy to investigate the robust hybridization gap in YbB${}_{12}$, revealing strong charge transfer and persistent gap features up to 200 K, with low-temperature magnetic correlations.

Contribution

It provides the first quantitative analysis of the hybridization gap in YbB${}_{12}$ using ultrafast spectroscopy, highlighting its robustness and electronic structure changes.

Findings

Hybridization gap persists up to ~200 K.

Strong charge transfer from 4f levels observed.

Low-temperature changes linked to antiferromagnetic correlations.

Abstract

In heavy fermions the relaxation dynamics of photoexcited carriers has been found to be governed by the low energy indirect gap, E$_{g}$, resulting from hybridization between localized moments and conduction band electrons. Here, carrier relaxation dynamics in a prototype Kondo insulator YbB${}_{12}$ is studied over large range of temperatures and over three orders of magnitude. We utilize the intrinsic non-linearity of dynamics to quantitatively determine microscopic parameters, such as electron-hole recombination rate. The extracted value reveals that hybridization is accompanied by a strong charge transfer from localized 4f-levels. The results imply the presence of a hybridization gap up to temperatures of the order of E$_{g}$/k$_{B}\approx200$ K, which is extremely robust against electronic excitation. Finally, below 20 K the data reveal changes in the low energy electronic structure, attributed to short-range antiferromagnetic correlations between the localized levels.