Hybridization gap formation in the Kondo insulator YbB12 observed using time-resolved photoemission spectroscopy

A detailed low-energy electronic structure of a Kondo insulator YbB12 was revealed by a synergetic combination of ultra-high-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 of T * similar to 110 K. In harmony with this, the latter discovered a very long recombination time exceeding 100 ps below similar to T *. 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.