We have been interested in the influence of defects on quasi-2D organic superconductors like kappa-(BEDT-TTF)₂X, in the framework of a review article [publi 23]. The BEDT-TTF term corresponds to the molecule bis(ethylenedithio)tetrathiafulvalenium while the symbol X is given for a monovalent anion such as Cu[N(CN)₂]Br (T_c = 12 K) or Cu(NCS)₂ (T_c = 11 K).

In the X = Cu[N(CN)₂]Br case, the anion ordering transition around 80 K induces some disorder in the conducting planes. The ratio of disordered phase remaining at low temperature increases with the sample cooling rate through the transition.

In a similar way to the (TMTSF)₂ClO₄ compound, the presence of anion disorder in these materials influences the superconducting phase properties, and particularly the physics of vortex pinning. We demonstrate that very often these materials behave like granular superconductors in which the magnetic measurements are dominated by the presence of Josephson junctions (or proximity effects), which radically change the interpretation of the experimental results.

We think that magnetic measurements - hysteresis cycles and low field susceptibility - may provide valuable information not only about the nature of the ordering transformation, but also about the vortex physics. Indeed we know that the vortex are very sensitive to the sample defects nature, geometrical shape and density. Which is interesting here is that it is possible to change the density of these kind of defects in a reproducible way, by annealing and cooling the material through the structural transformation and by playing with the cooling rate. This transformation also must influence the sample screening at very low field (H < H_c1) and at T << T_c.

FIG. 1. Kinds of disorder that can be encountered in organic conductors subjected to an anion ordering phase transition. - Left-hand side: after partial annealing close to the transformation temperature. Superconducting domains ("S", in yellow) are surrounded by grain boundaries representing the disordered phase at low temperature (in orange). - Middle: for a very long annealing time, only high temperature formed punctual defects remain. - Right-hand side: starting from the previous state, then rapidly quenching the sample, defects can be created with a size larger than the coherence length x.

Figure 1 displays three kinds of structural defects that may result from the 80 K transformation, in these organic superconductors. It is inspired by schemes once proposed in the case of (TMTSF)₂ClO₄ [publi 21].
So we have proposed several mechanisms enabling to account for the magnetisation evolution in the superconducting phase of these compounds [publi 23]. Also visit Sadok Senoussi's personal page.