The influence of chemical impurities and X-ray induced defects on the single-particle and spin-density wave conductivity in the Bechgaard salts

We present some recent experimental and theoritical results obtained on the single particle and spin-density wave (SDW) conductivity in the Bechgaard salts (TMTSF)₂NO₃ and (TMTSF)₂PF₆. Particular emphasis is paid to the clearest example (TMTSF)₂PF₆ for which there is experimental evidence, namely the absence od 2 kF diffuse scattering at low temperatures, for a pure SDW state below 12 K. A theoritical analysis starting from the anisotropic Hubbard model and taking into account the influence of long range Coulomb interactions can account for many features of the experimental data. The magnitude and temperature dependence of the threshold field and to some extent the magnitude of the SDW conductivity are well accounted for by this theory. Comparison with experimental data shows that for most batches of samples the SDW is weakly pinned to randomly distributed impurities or defects. Defects were then introduced in a controlled way by X-ray irradiation. This caused a substantial increase in threshold field and a changeover to the behaviour expected for a strongly pinned SDW. For a particular batch of samples the temperature dependence of the threshold field was unusual. A detailed theoretical analysis of commensurability pinning leads us to conclude that in this case the SDW was commensurate with the lattice. Some recent data dor the SDW conductivity in the field induced SDW phases of (TMTSF)₂PF₆ under pressure are also reported and discussed within the same theoretical framework.