Ab initio approach to exciton dynamics (Davide Sangalli)
Channel: Francesco Sottile
Date: 2022-04-13

In this webiar I present a fully ab initio approach to model the generation of non-equilibrium coherent excitonic states with ultra-short laser pulses. The modelling is achieved via the real-time propagation of the density matrix projected in the Kohn-Sham basis set, within the time{dependent Hartree plus Screened EXchange (TD-HSEX) approaximation [1].
I show how the generated density matrix can be used to model transient spectroscopy signals in the presence of strongly bound excitons. Using LiF as a prototype material, I show that the scheme is able to capture the exciton signature both in time-resolved angle-resolved photoemission
spectroscopy and transient absorption experiments [2, 3]. The approach is completely general and can become the reference scheme for modeling pump and probe experiment in a wide range of materials.
Finally I discuss how the generated coherent excitonic states can be related to non coherent states with a thermal distribution of excitons, and how to compute their lifetimes [4, 5].


[1] C. Attaccalite, M. Gruning, and A. Marini, Real-time approach to the optical properties of solids and nanostructures: Time-dependent bethe-salpeter equation, Phys. Rev. B 84, 245110 (2011).
[2] E. Perfetto, D. Sangalli, A. Marini, and G. Stefanucci, Pump-driven normal-to-excitonic insulator transition: Josephson oscillations and signatures of bec-bcs crossover in time-resolved arpes, Phys. Rev. Materials 3, 124601 (2019).
[3] D. Sangalli, Excitons and carriers in transient absorption and time-resolved arpes spectroscopy: An ab initio approach, Phys. Rev. Materials 5, 083803 (2021).
[4] H.-Y. Chen, D. Sangalli, and M. Bernardi, Excitonphonon interaction and relaxation times from rst principles, Phys. Rev. Lett. 125, 107401 (2020).
[5] D. Sangalli, E. Perfetto, G. Stefanucci, and A. Marini, An ab-initio approach to describe coherent and non-coherent exciton dynamics, The European Physical Journal B 91, 171 (2018).