A high-resolution photo-emission spectroscopic study was made of MgB2, and the opening of a superconducting gap with a narrow coherent peak was observed. It was found that the superconducting gap was s-like, with a gap value of 4.5meV at 15K. The temperature dependence (15 to 40K) of the gap value was of BCS form; suggesting that 2Δ/kTC at T = 0 was about 3. No pseudo-gap behavior was observed in the normal state. The present results strongly suggested that MgB2 could be categorized as being a phonon-mediated BCS superconductor in the weak-coupling regime.

High-Resolution Photoemission Study of MgB2. T.Takahashi, T.Sato, S.Souma, T.Muranaka, J.Akimitsu: Physical Review Letters, 2001, 86[21], 4915-7