Coexistence of Charge Order and Spin-Peierls Lattice Distortion in One-Dimensional Organic Conductors

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The electronic properties of quarter-filled organic materials showing spin-Peierls transition are investigated theoretically. By studying the one-dimensional extended Peierls-Hubbard model analytically as well as numerically, we find that there is a competition between two different spin-Peierls states due to the tetramized lattice distortion in the strongly correlated regime. One is accompanied by lattice dimerization which can be interpreted as a spontaneous Mott insulator, while the other shows the existence of charge order of Wigner crystal-type. Results of numerical density matrix renormalization group computations on sufficiently large system sizes show that the latter is stabilized in the ground state when both the on-site and the inter-site Coulomb interactions are large.