Ab initio excited state forces in the study of self-trapped excitons and coherent phonon generation.

In transition metal dichalcogenides (TMD) the exciton-phonon interaction (EPI) plays an important role in optical and Raman spectra. One manifestation of EPI is the excited state (ES) forces, which are the gradient of the ES potential energy surface. Those forces indicate the direction atoms tend to move due to light absorption and are useful in the microscopic understanding of exciton self-trapping. We implemented an ES forces code that combines GW/BSE results from BerkeleyGW and electron-phonon coefficients from Quantum Espresso. With our results, we can relax the system towards the minimum of the ES potential energy surface, using gradient descent or a harmonic extrapolation, and avoiding the need for 3N GW/BSE calculations for finite differences. First, we study LiF as a test system for self-trapped excitons (STEs). By relaxing from polaronic-like distortions we found an STE that shows absorption peaks redshifted about a few eV. This illustrates a novel and efficient approach to study STEs at an ab initio level. Then we move to MoS2. For the monolayer, we observe the coupling of A exciton with A’₁ phonon, while C exciton couples with the A’₁ and E’ modes. In the bilayer case, we also observe coupling with the layer breathing mode. Our results agree with experimental data on the generation of coherent phonons by ultrafast light pulses. This approach can be used as a guide to future investigations of EPI of diverse TMDs.