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Hybridization gap formation in the Kondo insulator YbB$_{12}$ observed using time-resolved photoemission spectroscopy

A detailed low-energy electronic structure of a Kondo insulator YbB$_{12}$ was revealed by a synergetic combination of ultrahigh-resolution laser photoemission spectroscopy (PES) and time-resolved PES. The former confirmed a 25-meV pseudogap corresponding to the Kondo temperature of this material, and more importantly, it revealed that a 15-meV gap and a Kondo-peak feature developed below a crossover temperature $T^\ast \sim 110$ K. In harmony with this, the latter discovered a very long recombination time exceeding 100 ps below $\sim$$T^\ast$. This is a clear manifestation of photoexcited carriers due to the bottleneck in the recovery dynamics, which is interpreted as a developing hybridization gap of a hard gap.