Spin and Charge Fluctuations near Metal-Insulator Transition in Dimer-Type Molecular Solid

Spin and charge fluctuations at 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 using the variational Monte Carlo method. Three kinds of the 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 the molecular dimerization and Coulomb interaction amplitudes, and is enhanced remarkably near the metal-insulator phase boundary, in which the spin fluctuation is almost unchanged. Implication for the $\kappa$-type BEDT-TTF salts is discussed.