Evidence for strong-coupling s-wave superconductivity in MgB2:: 11B NMR study -: art. no. 127001

We have investigated a gap structure in a newly discovered superconductor, MgB2, through measurement of the B-11 nuclear spin-lattice relaxation rate, (11)(1/T-1). (11) (1/T-1) is proportional to the temperature (T) in the normal state, and decreases exponentially in the superconducting (SC) state, revealing a tiny coherence peak just below T-c. The T dependence of 1/T-1 in the SC state can be accounted for by an s-wave SC model with a large gap size of 2 Delta /k(B)T(c) similar to 5 which suggests it is in a strong-coupling regime.