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

摘要

We performed Se and F-NMR measurements on single crystals of (TMTSF)2FSO3 tocharacterize the electronic structures of different phases in theTemperature-Pressure phase diagram, determined by precise transportmeasurements [Jo et al., Phys. Rev. B67, 014516 (2003)]. We claim that suchvarieties of electronic states in the refined phase diagram are caused bystrong couplings of the conduction electrons with FSO3 anions, especially withthe permanent electric dipoles on the anions. We suggest that as temperaturedecreases, the FSO3 anions form orientational ordering through two steps; firstonly the tetrahedrons form an orientational order leaving the orientations ofthe electronic dipoles in random (transition I); then the dipoles form aperfect orientational order at a lower temperature (transition II). In theintermediate temperature range between transitions I and II, we found anappreciable enhancement of homogeneous and inhomogeneous widths of 77Se-NMRspectrum. From the analysis of the angular dependence of the linewidth, weattributed these anomalies to the intramolecular charge disproportionation orimbalance and its slow dynamics caused by the coupling with the permanentelectric dipole of anion. Results of 19F-NMR relaxation and lineshapemeasurements support this picture very well. Electronic structures at higherpressures up to 1.25 GPa are discussed on the basis of the results of the 77Seand 19F-NMR measurements.