Theory of photoinduced phase transitions: From semiclassical to quantum aspects

We review recent progress of theoretical studies for the photoinduced phase transitions (PIPTs) to clarify what the PIPTs axe. There are two types of the PIPTs: (a) global change via optically excited states and (b) new material phase creation in optically excited states. First, concerning (a), photoinduced structural phase transitions via excited electronic states are discussed using a minimal one-dimensional model composed of localized electrons and lattices. We show that the global structural change by photoexcitation only at a single site is possible under the adiabatic or diabatic approximation. This dynamics of the domain boundaries (domain walls) is called the "photoinduced domino process," which is the photoinduced nucleation in nonequilibrium first-order phase transition. Second, concerning (b), we discuss quantum orders of electron-hole (e-h) systems, which are optically excited states of insulators consisting of many electrons and holes in two bands. In particular, the "exciton Mott transition," i.e., the "from-insulator-to-metal" transition of the e-h systems as the particle density increases is introduced. We stress that this transition depends strongly on dimensionality of the system.