Low-Energy Electronic Structure of the Kondo InsulatorYbB12

We have studied the low-energy electronic structure of a Kondo insulator ${\mathrm{YbB}}_{12}$ by high-resolution photoemission spectroscopy. A “Kondo peak” is observed $\sim 25$ meV below the Fermi level, which agrees well with the Kondo temperature, whereas the gap at the Fermi level is found much smaller, indicating that the magnetic properties at higher temperatures ( $\gtrsim$75 K) are indeed determined by the Kondo effect in spite of the gap formation at lower temperatures. A renormalized band picture is presented to describe the coexistence of the Kondo peak and the transport gap as well as the highly asymmetric line shape of the Kondo peak.