Spin Density and Quantum Fluctuations in Quasi-One-Dimensional Conductors

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The role of repulsive interaction in quasi-one-dimensional organic conductors has been studied in order to examine the fluctuation of spin density in low-dimensioanl electron system. Following three subjects have been investigated. (1) The fluctuation of the commensurate spin density wave (SDW) has been examined by calculating collective modes of a Hubbard model with a quarter-filled band. The longitudinal mode shows the gap and the coexistence of the 2k_F-SDW and the 4k_F-CDW.The effect of anion lattice was examined in terms of a phase Hamiltonian. (2) A role of interchain hopping in quasi-1D organic conductors was examined by applying the phase Hamiltonian based on the bosonization to two coupled chains with the repulsive interaction of backward and forward scatterings. The competition of SDW state and superconducting state explains the phase diagram of in the quasi-1D conductors. (3) The metal-insulatortarnsition in DCNQI-Cu salts was examined by applying the slave-boson mean-field theory to the periodic Anderson model with the electron-phonon (e-p) interaction. The fact that the M-I transition is the Peierls transition in the presence of the strong correlation has been shown by calculating the reentrant transition of M-I-M,the phase diagram specific heat and magnetic susceptibility, the dimensional effect and the antiferromagnetic state. Thus we have shown the mechanism of the M-I transition.