The excitonic insulator route through a dynamical phase transition induced by an optical pulse

TL;DR

This paper explores a dynamical phase transition in excitonic insulators triggered by optical pulses, highlighting the formation of Bose condensates, quantum oscillations, and self-trapping phenomena that lead to complex domain evolution.

Contribution

It introduces a unified framework for thermodynamic and dynamic effects in excitonic insulator phase transitions induced by optical excitation, emphasizing the role of exciton Bose condensates.

Findings

Macroscopic quantum oscillations observed due to interference effects.

Self-trapping of exciton wave functions leads to localized phase transformations.

Localized exciton density can trigger phase change even below average density threshold.

Abstract

We consider a dynamical phase transition induced by a short optical pulse in a system prone to thermodynamical instability. We address the case of pumping to excitons whose density contributes directly to the order parameter. To describe both thermodynamic and dynamic effects on equal footing, we adopt a view of the excitonic insulator for the phase transition and suggest a formation of the Bose condensate for the pumped excitons. The work is motivated by experiments in donor-acceptor organic compounds with a neutral-ionic phase transition coupled to the spontaneous lattice dimerization and to charge transfer excitons. The double nature of the ensemble of excitons leads to an intricate time evolution, in particular to macroscopic quantum oscillations from the interference between the Bose condensate of excitons and the ground state of the excitonic insulator. The coupling of excitons and the order parameter also leads to self-trapping of their wave function, akin to self-focusing in optics. The locally enhanced density of excitons 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 that evolve through dynamical phase transitions and sequences of merging.