Dynamical Phase Transition to the Excitonic Insulator State Induced by an Optical Pulse
We consider theoretically a dynamical phase transition induced by a short optical pulse in a system prone to a thermodynamic instability, particularly towards a charge-ordered state of electrons. We address the case of pumping to excitons whose density contributes additively to the thermodynamic order parameter like for charge-transfer excitons in neutral-ionic transitions. To describe both thermodynamic and dynamical effects on equal footing, we adopt for the phase transition a view of the “excitonic insulator” and suggest a formation of the macroscopic quantum state for the pumped excitons. The double nature of the ensemble of excitons leads to an intricate time evolution: the dynamical transition between number–preserved and phase–locked regimes, macroscopic quantum oscillations from interference between the Bose condensate of excitons and the ground state of the excitonic insulator. Modeling of an extended sample shows also stratification in domains of low and high densities which evolve through local dynamical phase transitions and a sequence of domains’ merging.