Theory of MX Chain Compounds - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 122

Fig. 8.9 Electron
configurations of various
states. The abbreviations of
STE, S 0 , S , and P are a self-
trapped exciton, a neutral
soliton, a positively or
negatively charged soliton,
and a positively or negatively
charged polaron, respectively

CDW

h ν

Exciton

STE

S 0

S 0

S + S -

P -

P +

CDW state in the top panel. Shining light will excite one electron from the deep
well (M 2+ site) to the shallow well (M 4+ site). As we have already discussed in
detail, the exciton state is a bound electron-hole pair. In this case, the exciton
resides at the two neighboring M sites as shown in the “Exciton” panel. We here
emphasize that the localized position of the exciton, namely, the position at
a particular pair of metals, i.e., M 4+ M 2+ , is just for convenience in drawing
the figure. The exact form of the exciton must obey the translational symmetry
and the spatial inversion symmetry (the inversion center is located at either the M 2+
site or the M 4+ site).
Following this excitation, we expect a self trapping of the exciton, that is, the
localization of the center of gravity of the exciton. Of course, this cannot occur
solely within the electronic degrees of freedom. We rather expect that a partial
melting of the halogen-dimerized pattern will cause such localization. In a sense,
this is very natural because the exciton, which is nothing but M 3+ M 3+ , prefers such
a lattice deformation, as is so in the Mott-insulator state. This state is usually called
a self-trapped exciton and abbreviated as “STE” (see the corresponding panel of
Fig. 8.9 ). From the exciton to the STE, the elevation of the M 2+ level and the
dropping of the M 4+ level occur, as shown in the top panel of Fig. 8.10 .
If we proceed with this process by one more step, we will expect a complete
inversion of the lattice pattern in this local area and a simultaneous electron
transition following this lattice movements, as is shown in the third panel of
Fig. 8.10 . Furthermore, we continue the process as described in the third and fourth