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β
U
M
V
MM
V
MXM
V
2
BCDW
M
2+
M
4+
X
2+
M
2+
X
−3
|y|
1.5
2
2
4
CDW
M
2+
M
2+
X
3+
M
3+
X
−
|y|
1
2.5
2
4
|y|
β
ACP
M
2+
M
3+
X
3+
M
2+
X
−
|y|
12 .54
CP
M
2+
M
3+
X
M
2+
M
3+
X
−
|y|
122 5
Fig. 12.5 Schematic structures of the electronic phases and the coefficients of their partial
energies per binuclear unit with respect to site-diagonal electron-lattice coupling
, on-site
repulsion
U
M
, nearest-neighbor repulsion
V
MM
,
V
MXM
, and next-nearest-neighbor repulsion
V
2
,
in the strong-coupling limit
b
12.6 Site-Diagonal Versus Site-Off-Diagonal Electron-Lattice
Interactions
The ACP phase is found at low temperatures in Pt
2
(RCS
2
)
4
I(R
CH
3
,
n
-C
4
H
9
)
[
10
,
11
], where the MMX chains are neutral. There is no counterion that acts as an
obstacle to displacements of M ions as in R
4
[Pt
2
(pop)
4
I]
n
H
2
O. Then, we set
K
MXM
to be zero. To start with, we simply ignore the long-range interactions and study the
competition between the site-diagonal (
¼
) electron-lattice
couplings and the effect of the on-site repulsion
U
M
on it. The site-off-diagonal
coupling
b
) and site-off-diagonal (
a
stabilizes the ACP phase, where the binuclear units are dimerized. It is
regarded as a spin-Peierls state because a singlet is formed on the nearest-neighbor
M sites accompanied with an X site in-between. Exactly diagonalizing the present
a
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