Mott transition in quasi-one-dimensional electron systems at quarter filling

Renormalization-group equations are derived and numerically solved for one-dimensional, quarter-filled electron systems with a backward scattering, the Umklapp process, and couplings with 0, 2kF, 4kF phonons. A charge gap opens when the Umklapp process becomes relevant through constructive interference with the coupling with 4kF phonons, though the Umklapp process itself is weak. Meanwhile, a spin gap opens when the coupling with 2kF phonons is strong enough. We consider three-dimensionality by cutting off the logarithmic divergence in the Cooper or 2kF electron-hole channel. The former case may explain a qualitative difference between the resistivity of (DMe-DCNQI)2Ag and that of (DI-DCNQI)2Ag.