Mechanism for photoinduced structural phase transitions in low-dimensional electron-lattice systems: Nonlinearity with respect to excitation density and aggregation of excited domains

We construct a model for photoinduced structural phase transitions, based on electron-lattice systems made of tunable number of chains. The ground state is chosen to be a charge-density wave at half filling. There the degeneracy of the ground states is lifted up in order to prepare two unequal states, namely, a stable one and a metastable one. The first important point is the nonlinearity, which means that more than one absorbed photon cooperatively drives the system into a semimacroscopic phase. In our model, we particularly assume that the system stays at a metastable state at the initial time. One absorbed photon tuned to the optical gap energy excites the electronic system to a charge-transfer state. After this photoexcitation, the system gradually relaxes to a self-trapped excitation state, but it still remains metastable because the conversion of the phase, namely, the reconstruction of charge distribution, is only local. With more than one absorbed photon, on the other hand, the channels to a global phase change open, and then the system changes into a stable ground state. Next, we study the condition that spatially separate excitations created by the photons aggregate with attractive forces. It is demonstrated by adiabatic potential-surface analyses and dynamical calculations that both the conditions, namely, those for the nonlinearity and the aggregation, are satisfied simultaneously in our model.