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
(Tc = 12 K) or
Cu(NCS)2 (Tc = 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 <
Hc1) and at T << Tc.
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
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.