Coherent-to-incoherent crossover in the ultrafast dynamics of electrons and phonons in photoexcited 2D materials*

First-principles simulations of photoexcited materials are extremely challenging due to the simultaneous interplay of light−matter, electron−electron, and electron−nuclear interactions. We here present a novel non-equilibrium Green's function method [1] based on the simultaneous inclusion of the GW [2], Ehrenfest, and Fan-Migdal [3] self-energies. The method accounts for quantum coherence and non-Markovian effects while treating electrons and nuclei on equal footing, thereby preserving fundamental conservation laws like the total energy. The impact of this advancement is demonstrated through real-time simulations of the complex multivalley dynamics in a molybdenum disulfide monolayer [4]. At high carrier density the energy exchange between electrons and phonons is very efficient, leading to a sizable increase of the lattice temperature within one picosecond. In this process, electronic coherence is lost, whereas lattice coherence endures for a significantly longer period. In this regime we also lay down the microscopic theory of coherent phonons interacting with excitons and provide the ab initio expression of the corresponding coupling [5]. The simulated transient absorption spectrum well reproduces the coherent oscillations of the exciton energies observed in recent experiments [6].

[1] G. Stefanucci, R. van Leeuwen, and E. Perfetto Phys. Rev. X, 13, 031026 (2023).

[2] E. Perfetto, Y. Pavlyukh, G. Stefanucci, Phys. Rev. Lett. 128, 016801 (2022).

[3] D. Karlsson, R. van Leeuwen, Y. Pavlyukh, E. Perfetto, and G. Stefanucci, Phys. Rev. Lett. 127, 036402 (2021).

[4] E. Perfetto and G. Stefanucci, Nano Letters 23, 7029 (2023).

[5] E. Perfetto, K. Wu and G. Stefanucci, submitted.

[6] C. Trovatello et al., ACS Nano 14, 5700 (2020).