Abstract
We theoretically study the lattice relaxation dynamics of a photogenerated single exciton which finally results in a macroscopic domain in insulating solids. Especially, we focus on three elementary nonlinear processes of this domain formation, which we call aggregation, growth and proliferation. This domain formation phenomenon is usually called `photoinduced structural phase transition' (PSPT). In these three elementary processes, the exciton does not stay within a single adiabatic potential surface but transits diabatically between the different potential ones. Hence, the full-quantum-mechanical treatment is indispensable. As one of the models for this type of phase transition, we consider a strongly coupled many exciton-Einstein phonon system. This is our relevant system. We also assume this system to interact with even and odd reservoir modes as well as photons. The even modes couple linearly with Einstein phonons and dissipate their energies, while the odd modes couple linearly with excitons and non-radiatively create and annihilate them. Furthermore, excitons are assumed to be strongly repulsive at the same sites, while attractive at the nearest-neighboring sites. The characteristic of our relevant system is the so-called multistable situation, where various multi-exciton states are energetically close to 1-exciton states. We also assume there is the third order anharmonic coupling between excitons, which naturally comes from the long-range Coulomb interaction and incomplete bosonic nature of excitons. Within the Markov approximation for reservoir modes and photons, the time evolution of the density matrix is numerically calculated. As a first work, we have shown characteristics of the early time dynamics of the formation of a domain, against various dissipation and relaxation processes. The importance of the aforementioned three elementary nonlinear processes are clarified. We conclude the multistability and the anharmonicity of exciton states are important for the occurrence of PSPT. We also conclude several ten excitons can be generated from one photogenerated exciton if these two characteristics are realized.
