High-resolution photoemission study of the temperature-dependentc−fhybridization gap in the Kondo semiconductorYbB12

The temperature-dependent metal-to-insulator transition in the Kondo semiconductor ${\mathrm{YbB}}_{12}$ 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 ${T}_{1}\ensuremath{\sim}150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and at the same time, the Yb 4${f}_{7∕2}$ Kondo peak at $55\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ grows and shifts to a lower binding energy. Below ${T}_{2}\ensuremath{\sim}60\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, an additional spectral feature at $15\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ becomes apparent in the Yb 4$f$ and Yb 5$d$ derived spectra, indicating a strongly hybridized character. The $15\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ feature in the Yb 4$f$ 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 4$f$ and Yb 5$d$ states.