We report inelastic light scattering from underdamped plasmons in azetidinium lead bromide (AzPbBr3). The plasmons are very strongly temperature dependent and serve as a soft mode for the semiconductor-insulator phase transition near TC {\guillemotright} 150 K, demonstrating a continuous decrease in hole concentration np(T) by at least a factor of four and implying a nearly tricritical transition. The plasmon frequency and linewidth agree with independent measurements, and the impedance analysis reveals a frequency dependence (modelled by a constant phase element, CPE) that can be identified as due to electron-phonon coupling. The dependence of plasmon frequency upon (TC-T) is analogous to that for magnons in magnetic insulators or soft transverse optical phonons in ferroelectrics and ferroelastics, or for phasons in incommensurately modulated insulators.
@article{arxiv.1904.07573,
title = {Plasmon Soft Mode in an Organic-Inorganic Hybrid Perovskite},
author = {Jiyu Tian and Karuna Kara Mishra and Eli Zysman-Colman and Finlay D. Morrison and Ram S. Katiyar and James F. Scott},
journal= {arXiv preprint arXiv:1904.07573},
year = {2019}
}