Angle-resolved photoemission study of novel boride compounds

We have performed high-resolution photoernission spectroscopy on newly discovered novel boride compounds, MgB2, La-doped CaB6, and DyB2C2. In MgB2, we observed the superconducting gap of about 4.5 meV with s-wave symmetry. In La-doped CaB6, we found a 1 eV energy gap at the X point in the Brillouin zone, in contrast with the LDA band calculation hut in good agreement with the GW calculation. This requests reexamination of the excitonic-insulator model based on an overlap of an electron and a hole band at the X point. In DyB2C2, we found two (one electron-like and one hole-like) Fermi surfaces, which are composed of the mixed states of the B2p. C2p and DySd orbitals, below the antiferro-quadrupolarsordering temperature.