Evidence for Strong-Coupling $\mathit{s}$-Wave Superconductivity in ${\mathrm{MgB}}_2$: ${}_{}{}^{11}{}_{}{}^{}B$ NMR Study

We have investigated a gap structure in a newly discovered superconductor, ${\mathrm{MgB}}_2$, through measurement of the ${}^{11}\mathrm{B}$ nuclear spin-lattice relaxation rate, ${}^{11}\left({1/T}_1\right)$. ${}^{11}\left({1/T}_1\right)$ is proportional to the temperature $\left(T\right)$ 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 $2\Delta {/k}_B{T}_c\sim 5$ which suggests it is in a strong-coupling regime.