Photoinduced dynamics of excitonic order and Rabi oscillation in the two-orbital Hubbard model

TL;DR

This paper explores how photoexcitation can enhance excitonic order in a two-orbital Hubbard model by analyzing Rabi oscillations, showing that this effect persists beyond simple models and is robust against quantum fluctuations.

Contribution

It demonstrates that photoinduced enhancement of excitonic order can be understood through Rabi oscillations and persists in realistic conditions including electron-phonon interactions and quantum fluctuations.

Findings

Rabi oscillation governs photoinduced excitonic enhancement.

The effect remains in models with finite transfer integrals.

Quantum fluctuations do not negate the enhancement mechanism.

Abstract

We investigate the condition for the photoinduced enhancement of an excitonic order in a two-orbital Hubbard model, which has been theoretically proposed in our previous work [Phys. Rev. B 97, 115105 (2018)], and analyze it from the viewpoint of the Rabi oscillation. Within the mean-field approximation, we simulate real-time dynamics of an excitonic insulator with a direct gap, where the pair condensation in the initial state is of BEC nature and the photoexcitation is introduced by electric dipole transitions. We first discuss that in the atomic limit our model is reduced to a two-level system that undergoes the Rabi oscillation, so that for single cycle pulses physical quantities after the photoirradiation are essentially determined by the ratio of the Rabi frequency to the pump-light frequency. Then, it is shown that this picture holds even in the case of nonzero transfer integrals where each one-particle state exhibits the Rabi oscillation leading to the enhancement of the excitonic order. We demonstrate that effects of electron-phonon interactions do not alter the results qualitatively. We also examine many-body dynamics by the exact diagonalization method on small clusters, which strongly suggests that our mechanism for the enhancement of the exctionic order survives even when quantum fluctuations are taken into account.