Angle-Resolved Photoemission Study of Novel Boride Compounds

We have performed high-resolution photoemission spectroscopy on newly discovered novel boride compounds, MgB 2 , La-doped CaB 6 , and DyB 2 C 2 . In MgB 2 , we observed the superconducting gap of about 4.5 meV with s -wave symmetry. In La-doped CaB 6 , we found a 1 eV energy gap at the X point in the Brillouin zone, in contrast with the LDA band calculation but 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 DyB 2 C 2 , we found two (one electron-like and one hole-like) Fermi surfaces, which are composed of the mixed states of the B2 p , C2 p and Dy5 d orbitals, below the antiferro-quadrupolar-ordering temperature.