Fermi surface of novel borides: MgB₂ and CaB₆

We have performed systematic high-resolution angle-resolved photoemission spectroscopy (ARPES) on two novel boride compounds, MgB₂ and CaB₆, to study the electronic structure near the Fermi level relevant to the anomalous physical properties (superconductivity and ferromagnetism) observed in these compounds. In MgB₂, we have succeeded in separately observing two superconducting gaps which originate in the σ and π bands, respectively, and thereby established the two-band superconductivity. ARPES measurements on AlB₂, which is a non-superconducting isostructural reference compound of MgB₂ with an additional electron in a unit cell, indicates the importance of the σ band for the superconductivity. In CaB₆, we observed an energy gap of 1 eV at the X point in the Brillouin zone, which suggests that CaB₆ is intrinsically a large-gap semiconductor. This observation requires reconsideration of the excitonic-insulator model to explain the anomalous ferromagnetism in CaB₆.