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 scat tering, the Umklapp process, and couplings with 0, 2k(F), 4k(F) phonon s. A charge gap opens when the Umklapp process becomes relevant throug h constructive interference with the coupling with 4k(F) phonons, thou gh the Umklapp process itself is weak. Meanwhile, a spin gap opens whe n the coupling with 2k(F) phonons is strong enough. consider three-dim ensionality by cutting off the logarithmic divergence in the Cooper or 2k(F) electron-hole channel. The former case may explain a qualitativ e difference between the resistivity of (DMe-DCNQI)(2)Ag and that of ( DI-DCNQI)(2)Ag.