Elemental ferroelectric topological insulator in ψ-bismuthene

Ferroelectric quantum spin Hall insulator (FEQSHI) exhibits coexisting ferroelectricity and time-reversal symmetry protected edge states, holding exciting prospects for inviting both scientific and application advances, particularly in two-dimensional systems. However, FEQSHI candidates that consist of only one constituent element are rarely reported. Here, we show that the ψ-bismuthene, an allotrope of bilayer Bi (110), as a concrete example of two-dimensional elemental FEQSHI. It is demonstrated that ψ-bismuthene possesses measurable ferroelectric polarization and nontrivial band gap with moderate switching barrier, making it highly suitable for the detection and observation of ferroelectric topologically insulating states. Additionally, the auxetic behavior, quantum transport property and ferroelectric controllable persistent spin helix in ψ-bismuthene are also discussed. These findings make ψ-bismuthene promising for both fundamental physics and technological innovations.