Category: News

Title | Driving Lattice Dynamics in 2D Perovskites via THz Kerr Effect

Date | October 2, 2023

Abstract | Charge carrier-phonon interaction governs lead halide perovskite’s optoelectronic properties. Understanding the complex vibrational dynamics and microscopic coupling mechanisms is crucial for material design. The THz-induced Kerr effect spectroscopy (TKE) technique allows us to drive the crystal lattice using sub-picosecond THz electric field pulses and observe the ensuing dynamics by probing the transient changes of birefringence. Recently, in 3D MAPbBr₃ at low temperature, we demonstrated coherent THz control of the octahedral twist modes, which potentially play an important role in dynamic charge carrier screening [1]. In this talk, I will introduce the technique and discuss our most recent studies on layered Ruddlesden-Popper HOIPs, which combine the intriguing properties originating from the soft, polar, and anharmonic lattice with confinement effects due to lowered dimensionality.  In quasi-2D compounds, already at room temperature we observe long-lived coherent phonon oscillations in the 0.5–3 THz range [2].  We analyze the mode symmetry and possible driving mechanisms and compare samples of different dimensionality to infer the microscopic origin of the vibrations.

References

[1]    M. Frenzel, M. Cherasse et al., Sci. Adv.9, eadg3856(2023) DOI:10.1126/sciadv.adg3856

[2]    Z. Zhang et al., Sci. Adv.9, eadg4417(2023) DOI:10.1126/sciadv.adg4417

Title | Anharmonic vibronic coupling in ultrasoft halide perovskites

Date | July 3, 2023

Abstract |First-principles simulations of halide perovskites typically assume that the potential energy felt by electrons is defined with the nuclei fixed at their crystallographic Wyckoff positions. This assumption misses the effect of local disorder (polymorphism) which affects profoundly the mechanical, optoelectronic, and light-absorbing properties of halide perovskites. In this talk, I will discuss the important role of polymorphism and anharmonicity in the electron-phonon coupling of halide perovskites [1]. In particular, I will demonstrate that (i) polymorphism is at the origin of overdamped and strongly coupled vibrational dynamics, (ii) anharmonic optical vibrations dominate thermal renormalization of their band gaps, and (iii) polymorphism is the key to understand the gradual variation of their band gaps around the phase transition temperatures. To address these points we develop a new very efficient methodology for anharmonic lattice dynamics, relying on the special displacement method (A-SDM) [2]. The A-SDM is a very simple tool that can be exploited by both condensed matter theorists and experimentalists, opening the way for systematic simulations of anharmonic phonons. Overall, our new theoretical advances [1,2] set up a new framework for interpreting the fundamental mechanisms driving the optoelectronic, transport, and photovoltaic properties of halide perovskites.

[1] M. Zacharias, G. Volonakis, F. Giustino, and J. Even, Anharmonic electron-phonon coupling in ultrasoft and locally disordered perovskites, arXiv:2302.09625 (2023).

[2] M. Zacharias, G. Volonakis, F. Giustino, and J. Even, Anharmonic lattice dynamics via the special displacement method, arXiv:2212.10633 (2023).

Acknowledgments : I acknowledge funding from the European Union (project ULTRA-2DPK / HORIZON-MSCA-2022-PF-01 / Grant Agreement No. 101106654). Views and opinions expressed are however those of the authors only and do not necessarily reflect those of the European Union or the European Commission. Neither the European 669 Union nor the granting authority can be held responsible for them. J.E. acknowledges financial support from the Institut Universitaire de France.