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Quasi-one-dimensional organic conductors (TMTTF)₂X salts exhibit various types of phase transitions such as magnetic ordering, charge ordering (CO), and superconducting transitions [1]. Among them, (TMTTF)₂SbF₆ shows a peculiar behavior under pressure: a cooperative reduction of CO and anti-ferromagnetic phase transition temperatures by the application of pressure has been reported by NMR measurements [2]. This result naively does not coincide with the case for typical CO transitions, where CO suppresses the tendency toward magnetic ordering due to decrease of the effective spin exchange coupling [3]. To explain this behavior, we investigate a 1/4-filled quasi-one-dimensional extended Hubbard model with inter-chain hopping and Coulomb interactions. In our model, the diagonal inter-chain hopping causes spin frustration between the dimers on the one-dimensional chains. By numerical exact diagonalization method, we calculate the charge and spin structure factors and investigate the relation between charge-ordering and inter-chain hopping. To compare the results with experiments, we determine the hopping parameters by first principles band calculation for several TMTTF salts and discuss the relation between spin frustration and CO.

[1] S. E. Brown, P. M. Chaikin, and M. J. Naughton, in “The Physics of Organic Superconductors and Conductors”, Ed. A. Lebed, Springer (2008)
[2] W. Yu et al., Phys. Rev. B. 70 (2004) 121101.
[3] Y. Tanaka and M. Ogata, J. Phys. Soc. Jpn. 74 (2005) 3283.