$(\mathrm{TMTTF}{)}_2{\mathrm{AsF}}_6$ undergoes two phase transitions upon cooling from 300 K. At $T_{\mathrm{CO}}=103\mathrm{}\hspace{0.5em}\mathrm{K}$ a charge ordering (CO) occurs, and at $T_{\mathrm{SP}}(B=9\mathrm{}\hspace{0.5em}\mathrm{T})=11\mathrm{}\hspace{0.5em}\mathrm{K}$ the material undergoes a spin-Peierls transition. Within the intermediate, CO phase, the charge disproportionation ratio is found to be at least 3:1 from ${}^{13}\mathrm{C}$ NMR $T_1^{-1}$ measurements on spin-labeled samples. Above $T_{\mathrm{SP}}$ up to about ${3T}_{\mathrm{SP}}$ $T_1^{-1}$ is independent of temperature, indicative of low-dimensional magnetic correlations. With the application of about 0.15 GPa pressure, $T_{\mathrm{SP}}$ increases substantially, while $T_{\mathrm{CO}}$ is rapidly suppressed, demonstrating that the two orders are competing. The experiments are compared to results obtained from calculations on the one-dimensional extended Peierls-Hubbard model.

• Received 3 May 2002

DOI:https://doi.org/10.1103/PhysRevB.66.081103