Anisotropic conductivity and charge ordering in (TMTTF)2X salts probed by ESR

Single crystal ESR data on (TMTTF)₂X salts are presented. Taking advantage of the large dimensions of the crystals, skin effect is studied through the asymmetry of the ESR resonance lines. When the microwave magnetic field is perpendicular to the stacking axis, a very rich temperature dependence of this asymmetry is found although the three components of the conductivity tensor often exhibit a semiconducting behavior. In particular, we show that anomalies are present when the TMTTF chains experience a Mott localization or a charge ordering. The whole set of data is discussed theoretically in relation with the anisotropy of the electrical conductivity. We show that the crystal orientation and crystal shape are crucial parameters in explaining the experimental results. More precisely, we found that at the charge ordering transition, the largest conductivity anomaly occurs along the transverse b direction, and we provide evidence, through ESR measurements of the conductivity in this direction, of a charge ordering transition at about 50K in (TMTTF)₂Br . With the same method, we also show that irradiation kills the charge ordering transition, while deuteration enhances its critical temperature.