Phase Diagram of the One-Dimensional Extended Hubbard Model at Quarter-Filling

We used exact diagonalization of small clusters and exact results in various limits to determine the phase diagram and the critical exponents of the one dimensional (1D) U-V model at quarter-filling. We found an instability of the Luttinger liquid to a charge-density wave (CDW) insulator across a boundary going from (U, V) = (+, 2t) to (4t, +). In the metallic phase, the dominant fluctuations are superconducting if V is large enough and spin-density wave (SDW) otherwise. In the latter case, the critical exponent of the momentum distribution does not exceed 9/16. The relevance of these results to 1D organic conductors is discussed.