Nonlinearity in the domain growth and macroscopic oscillations in photoinduced structural phase transitions

We study a two-chain electron–lattice (e–l) model, to know fundamental features of photoinduced structural phase transitions (PSPT). To avoid entering into unnecessarily specific details, the model is chosen to be as simple as possible, that is, it includes only nearest-neighbor hopping of electrons and a site-diagonal e–l coupling. Moreover, we prepare two states, that are, stable and metastable charge-density-wave (CDW) ground states and discuss stable-to-metastable transitions. In this article, we particularly focus the following two spectacular properties of PSPTs. One is the nonlinearity in the converted fraction as a function of the light intensity, while the other is the macroscopic oscillations of the photo-generated domains. The former has been noticed in a number of examples of PSPTs, and the latter has been found in TTF-CA, very recently. Performing adiabatic-potential-surface analyses and semi-classical dynamical calculations, we show that our model shows the above properties, to demonstrate that our model is suitable as a prototype describing the essences of PSPTs.