RENORMALIZATION-GROUP APPROACH TO THE METAL-INSULATOR TRANSITIONS IN (DCNQI)(2)M (DCNQI IS N,N'-DICYANOQUINONEDIIMINE AND M=AG, CU)

Metal-insulator transitions and different ground-state phases in the q uasi-one-dimensional materials, (R1R2-DCNQI)(2)M (R-1 = R-2 = CH3, I a nd M = Ag, Cu), are studied with a renormalization-group method. We us e one-dimensional continuum models with backward scatterings, umklapp processes, and couplings with 2k(F) and 4k(F) phonons (not static latt ice distortion). We take a quarter-filled band for M = Ag and a sixth- filled band coupled with a third-filled band for M = Cu. Depending on electron-electron and electron-phonon coupling strengths, the ground-s tate phase becomes a Tomonaga-Luttinger liquid or a state with a gap(s ). For M = Ag, there appears a spin-gap state with a dominant 2k(F) ch arge-density-wave correlation, a Mott insulator with a dominant 4k(F) charge-density-wave correlation, or a spin-Peierls state with differen t magnitudes of spin and charge gaps. Three dimensionality is taken in to account by cutting off the logarithmic singularity in either the pa rticle-particle channel or the particle-hole channel. The difference b etween the ground-state phase of the R-1 = R-2 = CH3 salt (spin-Peierl s state) and that of the R-1 = R-2 = I salt (antiferromagnetic state) is qualitatively I explained by a difference in the cutoff energy in t he particle-particle channel. For M = Cu, there appears a Mott insulat or with a charge-density wave of period 3 and a Peierls insulator with a charge-ensity wave of period 6. The conditions for the experimental ly observed, Mott insulator phase are strong correlation in the sixth- filled band, moderate electron-phonon couplings, and finite electron-4 k(F) phonon coupling. Resistance is calculated as a function of temper ature with a memory-function approximation in both cases above. It qua litatively reproduces the differences among the M = Ag and M = Cu case s as well as the R-1 = R-2 = CH3 and R-1 = R-2 = I cases.