CHARGE GAP IN THE ONE-DIMENSIONAL DIMERIZED HUBBARD-MODEL AT QUARTER-FILLING

We propose a quantitative estimate of the charge gap that opens in the one-dimensional dimerized Hubbard model at quarter-filling due to dim erization, which makes the system effectively half-filled, and to repu lsion, which induces umklapp scattering processes. Our estimate is exp ected to be valid for any value of the repulsion and of the parameter describing the dimerization. It is based on analytical results obtaine d in various limits (weak coupling, strong coupling, large dimerizatio n) and on numerical results obtained by exact diagonalization of small clusters. We consider two models of dimerization: alternating hopping integrals and alternating on-site energies. The former should be appr opriate for the Bechgaard salts, the latter for compounds where the st acks are made of alternating TMTSF and TMTTF molecules.