Spin and charge fluctuations in the vicinity of metal-to-Mott insulator transitions are studied in an organic solid with molecular dimers. The extended Hubbard model taking account of the internal electronic degree of freedom in a molecular dimer is analyzed by the variational Monte Carlo method. Three kinds of electronic phases, i.e., a metallic phase, an antiferromagnetic insulating phase, and a polar-charge ordered phase, compete with each other in the ground state. It is found that the polar-charge fluctuation is dominant in a wide range of molecular dimerization and Coulomb interaction amplitudes, and is markedly enhanced near the metal-insulator phase boundary, in which the spin fluctuation is almost unchanged. The implication for κ-type BEDT-TTF [bis(ehtylenedithio)-tetrathiafulvelene] salts is discussed.
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