Vibrational Structure and Electron-Phonon Interactions in Halide Perovskite CsPbBr₃

Inorganic halide perovskites, chief among them the bromine-based compound CsPbBr3, are promising materials for single photon and entangled photon pair generation in the form of quantum dots [1]. Since their performances are largely limited by the coupling of excitons to lattice vibrations, it is crucial to understand the phonon spectrum and electron-phonon interactions in these materials. Additionally, the absence of organic molecule makes inorganic perovskites ideal platforms to probe phonons and electron-phonon interactions and attain a finer understanding of carrier mobilities in hybrid perovskite thin films, as they lack the rotational degrees-of-freedom of the molecule that complicate the picture [2].

Here, we report inelastic neutron scattering measurements and ab initio calculations performed on CsPbBr3 to determine its vibrational spectrum and electron-phonon couplings [3]. Neutron scattering experiments are a powerful tool capable of probing the full angle-resolved phonon dispersion in the entire Brillouin zone. Measurements have been carried out on both the low-frequency phonons, yielding information on the elastic constants and structural phase transitions, and on the optic phonons dominating the coupling to carriers. State-of-the-art ab initio calculations of the phonon and inelastic neutron spectrum have been performed to determine the nature of the modes and to establish a link with Fröhlich models of electron-phonon interactions and polaronic excitons [4].

We observe a predominantly order-disorder phase transition featuring diffuse scattering rods, consistent with recent neutron and X-ray scattering studies [5-6]. Well-defined optic phonon bundles are measured unlike in certain hybrid perovskites (FAPbI3 and FAPbBr3) [7], although they exhibit a flat dispersion. Simulations point to a dominant Fröhlich coupling of charge carriers to a longitudinal optic mode around 20 meV, which is consistent with temperature broadening of photoluminescence spectra as well as Raman scattering measurements reported for CsPbBr3 quantum dots [8-9].

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