Quasi-one-dimensional bismuth halides offer an exceptional platform for exploring diverse topological phases, yet the nature of the room-temperature topological phase transition in Bi4I4 remains unresolved. While theory predicts the high-temperature β-phase to be a strong topological insulator (TI), experiments observe a weak TI. Here we resolve this discrepancy by revealing the critical but previously overlooked role of electron-phonon coupling in driving the topological phase transition. Using our newly developed ab initio framework for phonon-induced band renormalization, we show that thermal phonons alone drive β-Bi4I4 from the strong TI predicted by static-lattice calculations to a weak TI above ~180 K. At temperatures where β-Bi4I4 is stable, it is a weak TI with calculated surface states closely match experimental results, thereby reconciling theory with experiment. Our work establishes electron-phonon renormalization as essential for determining topological phases and provides a broadly applicable approach for predicting topological materials at finite temperatures.
@article{arxiv.2508.17268,
title = {Phonons Drive the Topological Phase Transition in Quasi-One-Dimensional Bi$_4$I$_4$},
author = {Wenjie Hu and Jiayi Gong and Yuhui Qiu and Lexian Yang and Jin-Jian Zhou and Yugui Yao},
journal= {arXiv preprint arXiv:2508.17268},
year = {2025}
}