Recent photoemission studies on MgB2 and related materials

MgB 2 shows the highest superconducting transition temperature among intermetallic compounds. Angle-resolved photoemission spectroscopy revealed their electronic structure, and the result is well explained within a simple band picture except for one band. Other photoemission studies on related materials of AlB 2 and CaAlSi also show an extra band similar to that in MgB 2 . The direct observation of the superconducting gap confirmed a two-gap superconductivity. The larger and smaller gaps open at the σ and π bands, respectively. The carbon substitution effect on the two gaps was evaluated by using sub-meV photoemission spectroscopy.