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electron-electron interaction. Thus the CP phase is reasonably converted to the
CDW phase. The optical conductivity spectrum changes accordingly.
In Pt
2
(RCS
2
)
4
I, the electric conductivity is rather high [
10
]. The itinerant
character of electrons is much stronger than the pop systems. The main difference
between the two systems is that MMX chains in R
4
[Pt
2
(pop)
4
I]
n
H
2
O are charged
while those in Pt
2
(RCS
2
)
4
I are neutral. The former systems need to have
counterions, which prohibit free modulation of the distances between the neighbor-
ing binuclear units. This strongly suppresses the appearance of the ACP phase. On
the other hand, the latter systems do not have counterions so that the distances
between the neighboring binuclear units are easily modulated by sufficiently strong
site-off-diagonal electron-lattice coupling. As a consequence, the ACP phase
appears. From the optical conductivity spectrum, it is evident that the transfer
integrals are large and electrons are more delocalized than in the pop systems.
A quantitative comparison between theoretical and experimental results is possible
by using a three-band model [
23
], which explicitly takes X p
z
orbitals into account,
instead of the present two-band model. Though the electric conductivity shows
“metallic” behavior in Pt
2
(CH
3
CS
2
)
4
I above room temperature, where the lattice is
not distorted, the optical conductivity spectrum shows a small but finite charge gap
[
10
]. This AV phase is thus regarded as a Mott-Hubbard insulator phase.
12.10 Photoinduced Dynamics in a Pop System
Generally, photoinduced phase transitions have attracted much attention [
25
]. They
are observed in a broad range of materials. Photoinduced phase transitions that are
realized experimentally and simulated theoretically include transitions from Mott
insulator to metal phases [
29
,
30
], charge-ordered insulator to metal phases in
quasi-one- [
31
,
32
] and two-dimensional [
33
-
35
] systems, ferroelectric ionic to
paraelectric neutral phases [
36
,
37
], and nonmagnetic to paramagnetic phases
[
38
-
41
]. Among MMX-chain compounds, a photoinduced transition from the
CDW phase to the CP phase has been found in a pop system, R
4
[Pt
2
(pop)
4
I]
n
H
2
O
[pop
P
2
O
5
H
2
2
,R
(C
2
H
5
)
2
NH
2
][
7
]. Its mechanism is studied by solving the
time-dependent Hartree-Fock equation in this and remaining sections of this
chapter. Above a threshold in the photoexcitation intensity, a transition indeed
takes place from the CDW to CP phases. The threshold intensity depends on the
relative stability of these phases, which can be explained qualitatively by their
diabatic potentials. However, the transition from the CP to CDW phases is hardly
realized for two reasons [
24
,
25
]: (1) low-energy charge-transfer processes occur
only within a binuclear unit in the CP phase; (2) it is difficult for the CDW order to
become long-ranged owing to its weak coherence.
We employ the extended Peierls-Hubbard model (
12.1
) again. Here, we set
1/
K
MXM
¼
¼
¼
irrelevant as
explained in the previous sections and ignore the long-range electron-electron
interactions,
V
MM
¼ V
MXM
¼ V
2
¼
0, which is appropriate for the pop system and makes
a
0 , for simplicity. Relabeling the site indices
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