Séminaire

Mardi 19 Mai 2026 à 11h00.

Ab initio excitonic properties and exciton-lattice interactions in semiconductors

Pierre LECHIFFLART
(Université de Twente)

Salle de séminaires Lippmann

Invité(e) par
Rajarshi SINHA ROY

présentera en 1 heure :

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Excitons are bound electron–hole pairs that play a central role in understanding and controlling the interaction between light and matter across a wide range of solid-state systems. In this seminar, I will discuss recent progress in unraveling excitonic properties and exciton–lattice interactions in two distinct classes of materials.

First, I will present my work on quasi-two-dimensional Ag/Bi double perovskites. These materials exhibit unexpected trends in exciton binding energies with increasing layer thickness, contrary to conventional intuition about dielectric and quantum confinement. We perform ab initio calculations within many-body perturbation theory, namely the GW approximation and the Bethe–Salpeter equation (BSE), on experimental and a series of model structures. We identify the mechanisms responsible for the unexpected excitonic binding energy trend, and investigate the effect of layer separation and stacking pattern, hinting at potential tunability of the excitonic properties.[1]

Second, I will turn to a recent ab initio approach for describing phonon-assisted luminescence in monolayer hexagonal boron nitride (hBN). This wide-band-gap 2D material hosts strongly bound excitons and strong exciton–phonon coupling. Our framework, based on a dynamical exciton–phonon self-energy that goes beyond the BSE, allows us to include both the direct and phonon-assisted emission channels observed experimentally in the luminescence spectra of monolayer hBN.[2]

Finally, I will touch upon my current project, which is to compute excited-state forces. Excitons create forces that can distort the crystal lattice locally, which in turn localize the exciton. This process results in self-trapped excitons, which are key to understand the emission spectra of certain materials. We based our approach on the BSE and derive an analytical formula that allows to compute gradients of the excited state total energy ab initio.[3]


[1] Lechifflart, P., Biega, R.I., Leppert, L., (2025) Structural Distortions Control Scaling of Exciton Binding Energies in Two-Dimensional Ag/Bi Double Perovskites, under review
[2] Lechifflart, P., Paleari, F., Sangalli, D., & Attaccalite, C. (2023). First-principles study of luminescence in hexagonal boron nitride single layer: Exciton-phonon coupling and the role of substrate. Physical Review Materials, 7(2), 024006.
[3] Del Grande, R. R., & Strubbe, D. A. (2025). Revisiting ab-initio excited state forces from many-body Green's function formalism: approximations and benchmark. arXiv preprint arXiv:2502.05144

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