K. Iwano, RELAXATION PROCESSES YIELDING NONLINEAR EXCITATIONS IN THE EXTENDED PEIERLS-HUBBARD MODEL FOR PHOTOEXCITED MX CHAINS, Journal of the Physical Society of Japan, 66(4), 1997, pp. 1088-1096
The relaxations occurring after photoexcitation in the MX chains are i
nvestigated, using a one-dimensional extended Peierls-Hubbard model. T
he special emphasis is placed on the processes in which the nonlinear
excitations such as solitons and polarons are created in the backgroun
d of the charge-density wave (CDW). The way of relaxations changes its
nature, depending mainly on the relative strengths of the on-site and
nearest-neighbor Coulombic repulsions U and V, respectively, to the o
n-site electron-lattice interaction S. When U or both L and V dominate
over S, soliton-pair states are unstable compared with the self-trapp
ed exciton (STE) in the localized Limit of the model. However, the neu
tral-soliton pair can be the lowest structural excitation in intermedi
ate-transfer cases. stabilized by the effect of itineracy. As for the
adiabatic potential surfaces, the uniform CDW is connected to the neut
ral-soliton pair, while the STE to the charged-soliton pair. It is exp
ected that the STE will make a transition to the neutral-soliton pair
nonadiabatically. When S dominates over U and V, on the other hand! th
e charged-soliton pair is the lowest structural excitation even in the
localized limit. In those cases, the uniform CDW is connected to the
charged-soliton pair, while the STE to the neutral-soliton pair on the
same potential surfaces. Following these considerations, the calculat
ion of the photoinduced absorption is also performed assuming that the
former case is realistic. The obtained spectrum in the presence of a
neutral soliton is compared with the experimental result.