Tuning the magnetic dimensionality by charge ordering in the molecular TMTTF salts

Low-dimensional (D) organic conductors TMTTF$_{2}$X are considered as a prototypical system for investigations of D-crossover phenomena, since the transfer integrals connecting the conduction chains are readily controlled by applied pressure [1]. In addition, recent NMR experiments in TMTTF salts imply the existence of another type of D-crossover which accompanies a multiferroic property; decreasing the ferroelectric-type charge ordering (FCO) by pressure is associated with the suppression of the antiferromagnetic (AF) transition temperature [2]. In this work, we theoretically investigate the interplay between FCO and magnetic states in TMTTF salts [3], and show that FCO increases 2-D AF spin correlation, whereas in the 1-D regime two different spin-Peierls states are stabilized. By performing first-principles band calculations for different salts and comparing our results with experiments, we identify the controlling parameters in the experimental phase diagram to be not only the inter-chain transfer integrals but also the amplitude of the FCO. [1] D. Jerome, Science 252, 1509 (1991). [2] W. Yu \textit{et al., }Phys. Rev. B. \textbf{70 }121101 (2004). [3] K. Yoshimi, H. Seo, S. Ishibashi, and S. E. Brown, arXiv:1110.3573 and arXiv:1110.3575.