Temperature dependence of the electronic states of Kondo semiconductor YbB12

The temperature dependences of the Yb 4f and 3d states of Kondo semiconductor YbB 12 single crystal have been studied by high-resolution photoemission spectroscopy taken at hν =100 and 5951 eV. The Yb 2+ 4f 7/2 state at the Fermi level is enhanced on cooling. With improved energy resolution, we discover an additional quasi-particle peak at 15 meV that developes below 60 K. From the Yb 3d spectrum at 22 K, the Yb valence is estimated to be∼2.88.