Metal-Insulator and Insulator-Insulator Transitions in the Quarter-Filled Band Organic Conductors

The theory of the $2k_F$ and $4k_F$ instabilities in quarter-filled band organic conductors is revisited. The phase angles of the $2k_F$ bond and charge density waves change as electron correlation is turned on, and this switching of the phase angle is critical for understanding the bond distortion patterns in the real materials. Intersite Coulomb interactions in the real materials must be nonzero but less than a critical value. Both intersite and intrasite charge density waves are destabilized in the quasi-two-dimensional regime for realistic parameters, explaining the weakening of these phases in the superconducting materials.