Three-dimensional (3D) topological ferroelectric (FE) insulators, in which topological and FE orders naturally coexist, enable field-controlled spintronic devices. In this work, we predict a new structure of bismuth monohalides Bi4Br4 and Bi4I4, denoted γ phase, and demonstrate that it is an ideal 3D topological FE insulator. Systematic first-principles calculations confirm the stability and synthesizability of γ-Bi4X4 (X=Br, I). Although the noncentrosymmetric γ phase crystallizes in the space group Cmc21 with no symmetry-based classifications/indicators, the nontrivial topology can be characterized by the spin Chern number (SCN). Spin-resolved Wilson loops show the sz SCN Csz=2, indicating the spin-resolved topology of a 3D quantum spin Hall insulator state. The z-direction polarization can be switched by interlayer sliding, requiring only crossing a small energy barrier. Finally, we design an electrically controlled spin-filter device on bilayer films that can generate a switchable spin-polarized current. Combining a single-phase crystal, a sizable band gap, and robust band topology against FE switching, these bismuth monohalides serve as a prototype of intrinsic 3D topological FE insulators, providing an ideal platform for realizing new nonvolatile functionalities in spintronic devices.
@article{arxiv.2604.23969,
title = {Three-dimensional topological ferroelectrics},
author = {Haohao Sheng and Sheng Zhang and Zhong Fang and Hongming Weng and Zhijun Wang},
journal= {arXiv preprint arXiv:2604.23969},
year = {2026}
}