NMR evidences of the coupling between conduction electrons and molecular degrees of freedom in the exotic member of the Bechgaard salt (TMTSF)₂FSO₃

We performed ⁷⁷Se- and ¹⁹F-NMR measurements on single crystals of (TMTSF)₂FSO₃ to characterize the electronic structures of different phases in the temperature-pressure phase diagram, determined by precise transport measurements [Jo et al., Phys. Rev. B 67, 014516 (2003)]. We claim that such varieties of electronic states in the refined phase diagram are caused by strong couplings of the conduction electrons with FSO₃ anions, especially with the permanent electric dipoles on the anions. We suggest that as temperature decreases, the FSO₃ anions form orientational ordering through two steps; first, only the tetrahedrons form an orientational order leaving the orientations of the electronic dipoles in random (transition I); then the dipoles form a perfect orientational order at a lower temperature (transition II). In the intermediate temperature range between transitions I and II, we found an appreciable enhancement of homogeneous and inhomogeneous widths of the ⁷⁷Se-NMR spectrum. From the analysis of the angular dependence of the linewidth, we attributed these anomalies to the intramolecular charge disproportionation or imbalance and its slow dynamics caused by the coupling with the permanent electric dipole of the anion. Results of ¹⁹F-NMR relaxation and lineshape measurements support this picture very well. Electronic structures at higher pressures up to 1.25 GPa are discussed on the basis of the results of the ⁷⁷Se- and ¹⁹F-NMR measurements.