NMR Study of $\kappa$-$\mathrm{(ET)_{2}Cu[N(CN)_{2}]Br}$

We report measurements of the $\mathrm{^{13}C}$ spin-lattice relaxation rate, $(T_{1}T)^{-1}$ of $\kappa$-$\mathrm{(ET)_{2}Cu[N(CN)_{2}]Br}$, an organic superconductor with $T_{C}=11.9\,\mathrm{K}$. The shape of the $(T_{1}T)^{-1}$versus $T$ graph suggests the existence of a finite spin singlet-triplet gap\footnote{C. P. Slichter, Experimental Evidence for Spin Fluctuations in High Temperature Superconductors, in \emph{Strongly Correlated Electronic Materials}: A Los Alamos Symposium, pp. 427-479, ed. K.S. Bedell \emph{et al}., Addison-Wesley (1994)}. Knight shifts were measured to search for a two-component electronic system. We also report measurements using an ``S-shape'' pulse sequence\footnote{L. R. Becerra, C. A. Klug, C. P. Slichter, and J. H. Sinfelt, J. Phys. Chem. \textbf{97}, 12014 (1993)} that inverts half of the absorption line, permitting one to study slow motions and spin density waves.