Observation of the coherence peak of 1H-NMR relaxation rate in the superconducting state of (MDT-TTF)2Aul2

We have investigated the superconductivity in the organic conductor, (MDT-TTF)2Aul2 (Tc = 4.1 K) from the temperature dependence of 1H nuclear relaxation rate, 1T1, measured by the field cycling method. The nuclear relaxation above 3 K is well characterized by a single relaxation time for nearly two decades. In the normal state, the standard Korringa relaxation holds. 1T1 just below Tc showed a clear enhancement, followed by a falling-off at lower temperatures. This behavior is just as predicted in the BCS theory (coherence peak) and implies that this material is a conventional superconductor with an isotropic gap. The decay of the nuclear magnetization below 3 K appeared non-sinle exponential. This is explained as the effect of trapped vortices.