K. Yonemitsu, RENORMALIZATION-GROUP APPROACH TO THE METAL-INSULATOR TRANSITIONS IN (DCNQI)(2)M (DCNQI IS N,N'-DICYANOQUINONEDIIMINE AND M=AG, CU), Physical review. B, Condensed matter, 56(12), 1997, pp. 7262-7276
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.