High-resolution photoemission study of the temperature-dependent c-f hybridization gap in the Kondo semiconductor YbB12

The temperature-dependent metal-to-insulator transition in the Kondo semiconductor YbB12 single crystal has been studied by means of high-resolution photoemission spectroscopy with a tunable photon energy. An energy gap in the valence band is gradually formed below T-1 similar to 150 K, and at the same time, the Yb 4f(7/2) Kondo peak at 55 meV grows and shifts to a lower binding energy. Below T-2 similar to 60 K, an additional spectral feature at 15 meV becomes apparent in the Yb 4f and Yb 5d derived spectra, indicating a strongly hybridized character. The 15 meV feature in the Yb 4f derived spectra is intense at the L point of the Brillouin zone and diminishes away from the L point. These results indicate that the energy gap is formed by the hybridization between the Yb 4f and Yb 5d states.