Theory of MX Chain Compounds - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 142

of electron correlation is not relevant in the determination of the charges. If the
readers carefully look at the domain wall area shaded in the figure, they will notice
that the left (right) region has more (less) electrons. We then move each domain
wall by one unit cell, and measure the increased or decreased dipole moments,
resorting to the method described in the preceding subsection. The obtained charges
are then found to be

0.46 e approximately, with e being the charge of the electron.
Thus, these values are fractional in the twofold meanings; first close to

(1/2) e and
then deviate from them with a substantial difference. We emphasize that the
approximate values of (1/2) e are easily explained in the localized limit by using
a figure like Fig. 8.16 , while the deviation from them depends on the parameter
value, being affected by the finite itinerancy.

8.4.4 Other Important Aspects of Photoconversions

At the end of this chapter, we briefly mention recent progresses on the
photoconversion in the MX-chains. By the way, this photoconversion is nowadays
called “photoinduced phase transition” in the field of optical properties of solids.
The MX-chains have particular research advantage among other popular materials
exhibiting photoinduced phase transitions because their optically excited states are
already clarified to almost satisfactorily levels except for a few remaining points
such as highly excited states in the Ni compounds, and therefore work well as
model materials. The progress worth being mentioned first of all is the ultrafast
nature of the domain formation. In the previous subsection, we described it based
on the adiabatic potential curves. Such a description assumes that important
dynamical changes, more specifically those in the electronic state, occur following
the change in the lattice system. This type way of thinking will be true if the lattice
system is the dominant driving force to determine the phases. Meanwhile, in our
CDW case, the effect of V also plays the essential role as we have already discussed.
For this reason, a concept of domain formation of purely electronic nature has been
pursued from both the experimental and theoretical sides. In particular, the obser-
vation of the ultrafast time scale in the early process of the CDW to Mott-Hubbard
conversion in the Pd compound is important, because it was much shorter than
80 fs, i.e., much less than the time scale of the relevant phonon frequency, about
0.1 ps. The contribution from the theoretical side was given with a quasi-one-
dimensional molecular solid, tetrathiafulvalene- p -chloranil (TTF-CA), as the target
material. Almost the same idea, namely, the ultrafast electronic domain formation,
was also presented in this material, and a theoretical calculation was performed
focusing on its optical conductivity spectra [ 50 ]. According to their results, the
excited states are characterized by excited domain states, and such tendency is more
conspicuous as the system goes closer to the phase boundary. By the way, TTF-CA
has two phases as electronic states, that are, the neutral phase and the ionic phase,
which are very similar to the CDW and Mott-insulator states, respectively, at least
from the theoretical point of view. Therefore, the results are applicable also to the