Dynamical patterns of phase transformations from self-trapping of quantum excitons

TL;DR

This paper models how short optical pulses induce phase transitions via exciton self-trapping, leading to domain formation and dynamical phase changes in systems with coupled excitonic and order parameters.

Contribution

It introduces a semi-phenomenological model describing spatio-temporal effects of exciton coupling to symmetry-breaking order parameters during phase transitions.

Findings

Self-trapped excitons can trigger phase transformations at sub-critical densities.

Formation of persistent domain structures after exciton recombination.

Dynamic interplay between excitons, charge transfer, polarization, and lattice deformations.

Abstract

Phase transitions induced by short optical pulses is a new mainstream in studies of cooperative electronic states. Its special realization in systems with neutral-ionic transformations stands out in a way that the optical pumping goes to excitons rather than to electronic bands. We present a semi-phenomenological modeling of spacio-temporal effects applicable to any system where the optical excitons are coupled to a symmetry breaking order parameter. In our scenario, after a short initial pulse of photons, a quasi-condensate of excitons appears as a macroscopic quantum state which then evolves interacting with other degrees of freedom prone to instability. This coupling leads to self-trapping of excitons; that locally enhances their density which can surpass a critical value to trigger the phase transformation, even if the mean density is below the required threshold. The system is stratified in domains which evolve through dynamical phase transitions and may persist even after the initiating excitons have recombined. We recover dynamic interplays of fields such as the excitons wave function, electronic charge transfer and polarization, lattice deformations.