The charge order (CO) state can be observed by microprobe measurements such as nuclear magnetic resonance (NMR) and infrared spectroscopy. We used the infrared spectroscopy to examine ${\left(\text{TMTTF}\right)}_2{\text{AsF}}_6$ and confirmed the CO transition seen in previous ${}^{13}\text{C}$-NMR studies. However we found the degree of charge disproportionation, $\Delta \rho =0.16$, to be smaller than that obtained from the analysis of $T_1$ in ${}^{13}\text{C}$-NMR and the redistribution of charge density below spin Peierls transition predicted by ${}^{13}\text{C}$-NMR was not observed at low temperature. We estimated the degree of spin disproportionation to be 0.11 from the analysis of the NMR shift in previous NMR studies. This small disproportionation does not require the redistribution of charge density at low temperature. The different behaviors of $T_1$ at charge-rich and charge-poor molecular sites observed by ${}^{13}\text{C}$-NMR can be examined by the contribution of the commensurate antiferromagnetic fluctuation with $4k_F$.

• Received 15 January 2010

DOI:https://doi.org/10.1103/PhysRevB.81.205107