Theory of MMX-Chain Compounds - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 249

Fig. 12.9 Schematic
electronic structures and
charge-transfer excitations in
the CDW and CP phases

CDW GS

X

P MM

P MXM
X X

CP GS

X

P MXM
X

P MM

X

a c

U M =2, V MM =1, V MXM =0.5

b

U M =4, V MM =2, V MXM =1 U M =6, V MM =3, V MXM =1.5

AV AV AV

CDW CDW CDW

CP CP CP

0 1 2 3 4 0 1 2 3 4

0 1 2 3 4

ω ω ω

Fig. 12.10 Optical conductivity in the AV, CDW, and CP phases of the 12-site model for
(a) U M ¼ 2, V MM ¼ 1, and V MXM ¼ 0.5, (b) U M ¼ 4, V MM ¼ 2, and V MXM ¼ 1, and
(c) U M ¼ 6, V MM ¼ 3, and V MXM ¼ 1.5. The other parameters are t MM ¼ 1, t MXM ¼ 0.8,
a ¼ 0.2, b ¼ 4, y 0 ¼ 0.1, and V 2 ¼ 0[ 23 ]

Among the energies of the experimentally observed peaks, that in the CDW
phase is lower than those in the CP phase. The energy difference between the
CDW and CP phases is reproduced in Fig. 12.10 for each parameter set. This
is easily understood in the strong-coupling limit. The low-energy charge-transfer
excitation takes place at E CDW
MXM

¼ bjyjV MM þ

2 V 2 in the CDW phase and at

2

E C MM ¼ 2 V 2 in the CP phase. We concluded above that the nearest-

neighbor repulsion through an X site V MXM is substantially weaker, at least for the

2 bjyjþV MXM