Theory of Photoinduced Phase Transitions: From Semiclassical to Quantum Aspects

We review recent progress of theoretical studies for the photoinduced phase tran- sitions (PIPTs) to clarify what the PIPTs are. 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 bound- aries (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 transi- tion,” 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.