Broken magnetic symmetry due to charge-order ferroelectricity discovered in (TMTTF)₂X salts by multifrequency ESR

We have investigated the charge-ordered state of the quasi-one-dimensional organic charge-transfer salts (TMTTF)₂X (where TMTTF stands for tetramethyltetrathiafulvalene and X = PF₆ AsF₆, SbF₆, and SCN) by performing comprehensive electron-spin-resonance (ESR) experiments at several frequencies for 4 K < T < 300 K. At elevated temperatures all compounds show a linear increase of ΔH(T ). Below the charge-ordering transition TCO important anomalies are observed in both the temperature dependence and the anisotropy of the ESR linewidth. In the case of the centrosymmetric anions PF₆, AsF₆, and SbF₆, the linewidth doubles its periodicity when rotated in a plane normal to the molecule axis; and it exhibits a significant frequency dependence. This enhanced linewidth is caused by anisotropic Zeeman interaction that we identify as a relaxation process in the charge-ordered state where magnetically inequivalent sites are present in adjacent stacks. Thus, charge order not only produces ferroelectricity but also breaks the symmetry of the magnetic degree of freedom in these organic quantum spin chains. For (TMTTF)₂SCN charge order coincides with the ordering of the non-centrosymmetric anions; the large contribution of dipolar interaction dominates the relaxation process.