Theory of MMX-Chain Compounds - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 255

Equations ( 12.6 ) and ( 12.7 ) show that, as the electron-lattice coupling b decreases,

) increases, and the barrier height E CDW ðbÞ the energy difference

D E (

b decreases in

the CDW phase. Finally, when the crossing point u c reaches /(2 K MX ), the CDW

phase becomes unstable and the transition to the CP phase is realized by infinitesi-
mal perturbations.

b

12.12 CP-to-CDW Transition

Starting from the metastable CP phase inside the hysteresis loop, we calculate the
time evolution for

> b c . Here, the CDW phase is thermodynamically
stable. Nonetheless, the CDW phase is hardly photoinduced. When four electrons
are excited, the initial CP domain acquires a few local defects. But the defects do
not proliferate. In them, one of the four orders (i.e., positive

b ¼ 5.5

r CDW , negative r CDW ,

positive r CP ) is locally developed, but they just stay there. In the

defect-free CP regions, the CP order is slightly weakened initially, but it is almost
completely recovered. The CP phase is dynamically stable. Even when more
electrons are excited, this situation is not altered.
The difference between the dynamics from the CDW phase and that from the CP
phase is at first understood on the basis of how charge is transferred by
photoirradiation. In the CDW phase, charge transfer shown in Fig. 12.9 among
the binuclear units needs a lower energy than that within the unit because the latter
costs on-site repulsion U M . In the CP phase, the situation is opposite, as long as t MM
is larger

r CP , and negative

than t MXM . This fact holds even if long-range electron-electron

interactions are present. Recall that E CDW
MXM MM and E C MXM >E C MM in the strong-

coupling limit. Therefore, by lower energy photoirradiation of the CP phase, charge
is transferred only within each binuclear unit, so that the charge density does not
become disproportioned among the binuclear units. Thus the path to the CDW
phase is closed in the low-energy excitation channel.
Thus, we become interested in what happens if the CP phase is irradiated by
higher energy photons? Again starting from the CP phase at b ¼ 5.5 > b c ,we
excite four electrons now from the lowest occupied orbitals to find that the initial
CP domain is quickly destroyed. There appear CDW domains with positive and
negative

<E CDW

r CDW . They occupy most of the areas, but none of the CDW domains
dominates over the others. The boundaries between different CDW domains sur-
vive. Charge transfer among the binuclear units takes place incoherently. Since the
interunit charge transfer is energetically unfavorable, the CDW domains do not
proliferate.
Next, we artificially added interunit repulsion V MXM between the nearest-
neighbor sites bridged by a halogen site, without introducing intraunit nearest-
neighbor repulsion V MM . In real materials V MM is larger than V MXM . Since V MXM
increases the energy of the intraunit charge-transfer excitation only, the energy of
the interunit charge-transfer excitation becomes relatively lower in the CP phase
also. Now the path to the CDW phase appears open in the low-energy excitation