Two-dimensional organic superconductors studied by NMR under pressure

The (BEDT)₂X family of layered superconductors is one of the largest among nearly 50 organic superconductors currently known. One of the advantages of the organic compounds as prototype materials for studying the mechanisms of superconductivity is their relatively high sensitivity to hydrostatic pressure. We review recent NMR studies of these compounds using NMR under liquid and gas pressure. We focus on the low temperature part of the phase diagram where the physics is controlled by electronic correlations leading to a competition between magnetism and superconductivity. This interplay between different ground states is shown by the observation of a pseudo-gap and antiferromagnetic fluctuations and can be finely tuned by the application of pressure. Using a gas pressure system gives the unique possibility of sweeping the pressure at low temperature. Recently we used this technique to study the AF-SC boundary and established the existence of a first order transition line between the superconducting and antiferromagnetic states. This revised version was published online in July 2006 with corrections to the Cover Date.