  Using machine learning methods, we explore different types of domain walls in
the recently unveiled single-element ferroelectric, the bismuth monolayer
[Nature 617, 67 (2023)]. Remarkably, our investigation reveals that the charged
domain wall configuration exhibits lower energy compared to the uncharged
domain wall structure. We also demonstrate that the experimentally discovered
tail-to-tail domain wall maintains topological interfacial states caused by the
change in the Z_2 number between ferroelectric and paraelectric states.
Interestingly, due to the intrinsic built-in electric fields in asymmetry DW
configurations, we find that the energy of topological interfacial states
splits, resulting in an accidental band crossing at the Fermi level. Our study
suggests that domain walls in two-dimensional bismuth hold potential as a
promising platform for the development of ferroelectric domain wall devices.
