Structures and Optical Properties of Pt and Pd Compounds with Charge-Density-Waves - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 8

Chapter 2
Structures and Optical Properties of Pt and Pd
Compounds with Charge-Density-Waves

Hiroshi Okamoto and Hiroyuki Matsuzaki

2.1 Introduction

Dynamics of excitons and photocarriers in one-dimensional (1D) semiconductors have
been attracting much attention from both theoretical and experimental point of views.
In 1D electronic systems, it is known that the excitonic effect is remarkably enhanced
due to the singularity of the 1D Coulomb potential. The effect of the electron-lattice
(e-l) interaction is also important in 1D electronic systems as exemplified by the Peierls
transition. It was theoretically expected that a free exciton (FE) is easily relaxed to a
self-trapped exciton (STE) via lattice relaxations, since there is no barrier between an
FEandanSTEin1Dsystems[ 1 ]. In 1D half-filled electronic systems, it is known that
the e-l interaction sometimes plays a significant role on their electronic structures and
charge dynamics. A typical example of such an e-l coupled system is trans -
polyacetylene ( trans -(CH x )), in which the dimerization or equivalently the bond
alternation occurs via the Peierls mechanism [ 2 ]. In trans -(CH) x ,itisknownthat
charge and/or spin carriers are stabilized as solitons [ 3 , 4 ] or polarons under the
influence of the e-l interaction, which exhibit characteristic midgap absorptions.
Thus, clarifying the nature of solitons and polarons as well as of excitons is essential
for understanding optical and transport properties of the 1D e-l coupled systems. In
general, conjugated polymers including (CH) x cannot be obtained as single crystals and