$^{13}$C NMR Study of Slow Motions in $\kappa$-(ET)$_2$Cu[N(CN)$_2$]Br

Like the high-$T_{C}$ cuprates, the 2D organic superconductor $\kappa$-$\mathrm{(ET)_{2}Cu[N(CN)_{2}]Br}$ ($T_{C}=11.9$~K) exhibits a pseudo-gapped phase above the superconducting transition, as indicated by the $^{13}\mathrm{C}$ spin-lattice relaxation rate ($1/T_{1}T$) peak at about 50~K. While $^{13}\mathrm{C}$ NMR has been used extensively to probe the pseudo-gapped regime, $T_{1}$ is only sensitive to fast motions in the MHz scale (Larmor frequency), and $T_{2}$ remains relatively constant in the pseudo-gapped regime. Neither $T_{1}$ nor $T_{2}$ give us any clue about any possible slow motions. We report measurements using the stimulated echo pulse sequence\footnote{L. R. Becerra, C. A. Klug, C. P. Slichter, and J. H. Sinfelt, J. Phys. Chem. \textbf{97}, 12014 (1993).} which is capable of providing more detailed information on possible slow motions in the pseudo-gapped regime.