Theory of MMX-Chain Compounds - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 256

Fig. 12.13 Schematic
processes that recover the
coherence of the respective
orders. To convert r CDW < 0
into r CDW > 0, two electrons
need to be transferred to the
next-nearest-neighbor site,
which is beyond the halogen
site. To convert r

CDW CP

X X X X X X

< 0 into CP

X X X X X X

r 0, only one electron

needs to be transferred to the
nearest-neighbor site within
the binuclear unit [ 24 ]

> CP

X X X X X X

X X X

X X X

channel because proliferation of CDW domains is no longer energetically hindered.
In spite of this favorable fact, the transition to the CDW phase is still hardly
realized.
Suppose CP (CDW) domains with positive and negative

r CDW ) neighbor
with each other. A unit of the CP domain consists of one binuclear unit. To convert
r CP <

r CP (

0, only one electron needs to be transferred to the nearest-
neighbor site within the binuclear unit, as shown in Fig. 12.13 . Meanwhile, a unit of
the CDW domain consists of two binuclear units. To convert

0 into r CP >

r CDW < 0 into

r CDW > 0, two electrons need to be transferred to the next-nearest neighbor site

beyond the halogen site. Therefore, a wrong phase is easily corrected in the CP
phase, helping the growth of a single-phase CP domain. The coherence of the
charge-lattice order is easily recovered in the CP phase. In other words, it is
much easier for the CP order to become long ranged than for the CDW order.
Here, the tunneling amplitude connecting the anti-phase domains is important. The
barrier between the CP domains is much lower than that between the CDW
domains. The tunneling process is assisted by large transfer integrals. In fact,
when we reduce the interunit transfer integral t MXM , we can easily suppress the

transition from the CDW to CP phases also.