Observation of the coherence peak of /sup 1/H/NMR relaxation rate in the superconducting state of (MDT-TTF)/sub 2/AuI/sub 2/

Summary form only given. It has been argued whether the symmetry of superconducting gap of the organic superconductor, (BEDT-TTF)/sub 2/X (X=Cu(NCS)/sub 2/, Cu[N(CN)/sub 2/]Br) is conventional BCS-like or not, but the results are controversial thus far. In principle, the temperature dependence of NMR relaxation rate, 1/T/sub 1/, can probe the type of superconductivity. In the (BEDT-TTF)/sub 2/X, vortex dynamics have contributed to the relaxation and masked the hyperfine coupling with conduction electrons. We have found that the hyperfine coupling of /sup 1/H nuclei in the MDT-TTF molecule is much larger than that of the other organic molecule. We have investigated the temperature dependence of /sup 1/H-1/T/sub 1/ in the organic superconductor (MDT-TTF)/sub 2/AuI/sub 2/ (T/sub c/= 4.1 K), by applying the field cycling technique. The nuclear relaxation at zero field both in the normal and superconducting states (above 3K) is well characterized by a single relaxation time over one decade. In the normal state 1/T/sub 1/ followed the standard Korringa relation. I/T/sub 1/, just below T/sub c/, showed a clear enhancement as predicted in the BCS theory (´coherence peak´) and was followed by a falling off at lower temperatures. These results imply that the title material is a conventional superconductor of a BCS type. The decay of the nuclear magnetization below 3K appeared non-single exponential. This is, explained as the contribution of creeping motion of vortices.