Direct observation of superconducting gaps in MgB2 by angle-resolved photoemission spectroscopy

High-resolution angle-resolved photoemission spectroscopy has been carried out to clarify the anomalous superconductivity of MgB2. We observed three bands crossing the Fermi level, which are ascribed to B2p-sigma, pi and surface bands. We have succeeded for the first time in directly observing the superconducting gaps of these bands separately. We have found that the superconducting-gap sizes of sigma and surface bands are 6.5+/-0.5 and 6.0+/-0.5 meV, respectively, while that of the pi band is much smaller (1.5+/-0.5 meV). The present experimental result unambiguously demonstrates the validity of the two-band superconductivity in MgB2.