Temperature-Dependent High-Resolution Photoemission Study of the Kondo Insulator YbB12

We have performed a detailed temperature-dependent photoemission study of the conduction-band states in the Kondo insulator ${\mathrm{YbB}}_{12}$ using single crystalline samples. Measurements with improved energy resolution have revealed (pseudo)gaps of two energy scales. The size of the narrower gap $\sim 10\mathrm{meV}$ and its disappearance at high temperatures are consistent with the temperature dependence of the electrical resistivity and the magnetic susceptibility. The larger pseudogap of $\sim 100\mathrm{meV}$ depends on temperature even at room temperature. We discuss how the characteristics of the Kondo insulator are reflected in the conduction-band photoemission spectra.