A possible mechanism for photoinduced structural phase transitions in low-dimensional electron-lattice systems

We study low-dimensional electron-lattice (el-l) models in order to know fundamental features of photoinduced structural phase transitions (PSPT). To avoid entering into unnecessarily specific derails, the model is chosen to be as simple as possible, namely, includes only nearest-neighbor hopping of electrons and a site-diagonal cl-l coupling. Our special concern is in the geometrical and dimensional effects and hence single- and coupled-chain systems as well as a two-dimensional system are investigated. We prepare two states, that are, stable and metastable charge-density-wave (CDW) ground states and discuss metastable-to-stable and stable-to-metastable transitions. For both the cases, we particularly focus on two important features. One is the nonlinearity in the converted Fraction as a function of the light intensity, while the other is the aggregation of excitations. Performing adiabatic-potential-surface analyses and dynamical calculations, we demonstrate that our model changes its nature drastically when the geometry and the dimensionality are altered, and then manifests both the features simultaneously with suitably adjusted conditions. (C) 2000 Elsevier Science B.V. All rights reserved.