NMR in the 2 D Organic Superconductors

We review recent NMR studies of the layered organic superconductors of the (BEDT)2X family. First the normal state properties are discussed. The central issue is the importance of the interplay between antiferromagnetism and superconductivity in these materiais demonstrated by the observation of a pseudogap developing prior to the superconducting transition. An extended analysis of the Knight shift and the nuclear relaxation rate gives an evidence for strong antiferromagnetic fluctuations enhanced by Coulomb repulsion and the nesting of the Fermi surface. The NMR data show some analogy with High Tc superconductors however the comparison is not straightforward as there is no clear evidence for spin-charge decouphng. The use of high pressure to tune the amplitude of AF fluctuations enables us to draw a general phase diagram of the family. In the second part the NMR experiments in the superconducting state are presented. Une of the fundamental questions addressed by these experiments is the symmetry of the order parameter related to the painng mechanism. The most spectacular NMR measurements are those performed in the lock-in state, which grue access to the density of superconducting excitations at low energy. These expenments suggest that the pairing state may be unconventional featunng an anisotropic superconducting gap, possibly with nodes. Finally, the NMR contribution to the understanding of the nature of the vortex state is discussed in the context of recent results on ~-(ET)2Cu[N(CN)2]Br where the ~H relaxation was used as a probe of the vortex density fluctuations.