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

TL;DR

This study uses NMR techniques to reveal how conduction electrons in (TMTSF)2FSO3 interact with the electric dipoles of FSO3 anions, showing complex phase transitions influenced by electron-anion coupling.

Contribution

It provides the first detailed NMR evidence of coupling between conduction electrons and molecular dipoles in (TMTSF)2FSO3, elucidating the nature of its phase transitions.

Findings

Identification of two-step orientational ordering of FSO3 anions.

Observation of charge disproportionation linked to anion dipole dynamics.

Evidence of electron-anion coupling affecting electronic phases.

Abstract

We performed Se and F-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. B67, 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 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 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.