The gap structure was investigated via measurements of the 11B nuclear spin-lattice relaxation rate. The latter was proportional to temperature in the normal state, and decreased exponentially in the superconducting state; revealing a tiny coherence peak just below TC. The temperature dependence of the relaxation rate in the superconducting state could be explained by an s-wave superconducting model with a gap size of about 5; suggesting that it was in a strong-coupling regime.

Evidence for Strong-Coupling s-Wave Superconductivity in MgB2 - (11)B NMR Study. H.Kotegawa, K.Ishida, Y.Kitaoka, T.Muranaka, J.Akimitsu: Physical Review Letters, 2001, 87[12], 127001 (3pp)