Theory of MX Chain Compounds - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 124

2.9

2.7

STE

2.5

S +-

1.5

2.3

1

0

0.5

5

10

0

15

Fig. 8.12 Adiabatic potential surface of the first singlet excited state. Reprinted from Iwano [ 24 ]

3

2

S S

0 0

CDW

1

1.5

0

1

5

0.5

10

15

0

20

Fig. 8.13 Adiabatic potential surface of the ground state (singlet). Reprinted from Iwano [ 24 ]

two typical patterns of which are illustrated in Fig. 8.11 . As is seen in the figure,
these patterns continuously connect a local deformation like the STE and a kink-
anti-kink structure with an inverted region in between as the structures of the
solitons. Regarding the calculation method of the electronic states, we perform
almost the same type of calculation that we introduced in the previous subsection,
namely, the HF calculation for the ground state and the single-CI calculation for the
excited states, although the lattice configuration is fixed and not optimized in
this case. Using the same parameter set as that in the previous sections, namely,
( U , V , S , t 0 ) (2.0, 1.2, 0.27, 1.3), in Figs. 8.12 and 8.13 we show adiabatic

potential surfaces as functions of the degree of deformation,

¼

DQ , and the
soliton-soliton distance, l 0 . The amplitude of the regular dimerization, q 0 ,isof
course determined beforehand. The width of each soliton, w , is optimized at each
point of ( DQ , l 0 ). Looking at Fig. 8.12 , we now judge that the energy of the charged

soliton pair is higher than that the STE in this parameter set. Moreover, inspecting
both the two surfaces, we find that the STE is located higher than the neutral soliton
pair. These features are understood within a general trend [ 24 ], although we do not
enter into the details here. The cross section at

DQ ¼ 1.0 in the next figure is placed

to make these features more transparent.