Robust hybridization gap in the Kondo insulator YbB12 probed by femtosecond optical spectroscopy

In heavy fermions the relaxation dynamics of photoexcited carriers has been found to be governed by the low energy indirect gap ${\mathrm{E}}_{g}$ resulting from hybridization between localized moments and conduction band electrons. Here, carrier relaxation dynamics in a prototype Kondo insulator $\mathrm{Yb}{\mathrm{B}}_{12}$ is studied over a large range of temperatures and over three orders of magnitude. We utilize the intrinsic nonlinearity 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 pres…