Tuning the Magnetic Dimensionality by Charge Ordering in the Molecular TMTTF Salts

We theoretically investigate the interplay between charge ordering and magnetic states in quasi-one-dimensional molecular conductors ${\mathrm{TMTTF}}_{2}X$, motivated by the observation of a complex variation of competing and/or coexisting phases. We show that the ferroelectric-type charge order increases two-dimensional antiferromagnetic spin correlation, whereas in the one-dimensional regime two different spin-Peierls states are stabilized. By using first-principles band calculations for the estimation for the transfer integrals and comparing our results with the experiments, we identify the controlling parameters in the experimental phase diagram to be not only the interchain transfer integrals but also the amplitude of the charge order.

Figure 1

A schematic phase diagram for TM salts [4]. The ambient pressures for ${\mathrm{TMTTF}}_2{\mathrm{SbF}}_6$ and ${\mathrm{TMTTF}}_2{\mathrm{PF}}_6$ are shown. DM, FCO, AFM, SP, and SC represent dimer-Mott, ferroelectric charge ordering, antiferromagnetic, spin-Peierls, and superconducting states, respectively. The dashed line is a crossover, while the solid lines are phase transitions. The inset shows the arrangement of TM molecules in the conducting plane.

Figure 2

Interdimer and intradimer charge and spin structure factors $C_{\pm }(\mathit{q})$ and $S_{\pm }(\mathit{q})$ at (a) $V=0$ and (b) $V=2$, for $U=4$ and $t_b=0.1$. (c) $C_{-}(0,0)$ (background color) and $S_{+}(\pi ,\pi )$ (contour) on the $(t_b,V)$ plane: The two-dimensional AFM correlation is developed by both $t_b$ and $V$. (d) The leading spin-exchange process from the fourth-order perturbation in the presence of FCO.

Figure 3

(a) $V$ dependence of the order parameters in the charge densities and lattice distortions for $U=4$, $t_b=0.1$, $K_1=0.8$, and $K_2=1$. (b) Ground state phase diagram on the $(t_b,V)$ plane for $U=4$, $K_1=0.8$, and $K_2=1$. “2DAFM” stands for the enhanced two-dimensional AFM correlation, while the other abbreviations (see the text) represent ordered phases. The proposed trajectory for the pressure axis in the phase diagram Fig. 1 is shown by the arrow.