Professor Deng Junkai of the State Key Laboratory for Mechanical Behavior of Materials at Xi'an Jiaotong University (XJTU), in collaboration with Professor Liu Zhe of the University of Melbourne, has used deep potential molecular dynamics (DeePMD) simulations to discover a novel ferroelectric topological domain structure composed of intertwined domain pairs in elemental bismuth (Bi) monolayers, and further investigated its topological robustness and domain wall evolution under mechanical loading.

In ferroelectric materials, domain walls – the critical interfaces separating regions with different polarization orientations – dictate functional properties such as conductivity and electromagnetic coupling.

Traditionally, domain walls in ferroelectric materials favor head-to-tail, electrically neutral configurations to minimize electrostatic energy. However, the recent discovery of intrinsically stable 180°-charged domain walls in elemental Bi monolayer (a 2D ferroelectric) has challenged this convention, opening new avenues for fundamental research and functional device applications.

Because electronic polarization in 2D bismuthene is relatively weak, the domain wall energy is dominated by strain energy rather than the electrostatic energy typical of traditional ferroelectrics. This shift in the energy balance renders 180°-charged domain walls the most stable configuration in this system.

Consequently, understanding and manipulating the dynamic behavior of these charged domain walls and their resulting topological domain structures at finite temperatures has become a key challenge for functional applications.

By precisely controlling simulation conditions, the team explored the thermodynamic and kinetic characteristics of four typical domain walls (charged/neutral 90° and 180° walls) at finite temperatures.

They revealed that 180°-charged domain walls undergo a unique shuffle-type polarization-switching mechanism, whereas 90° domain walls follow the traditional bond-breaking and bond-forming mechanism.

Through high-temperature annealing simulations, the team identified a unique checkerboard topological domain structure consisting of intertwined domain pairs (i.e., sets of ferroelectric domains with opposite polarization).

In this architecture, 180°-charged domain walls dictate the formation of two sets of orthogonal ferroelectric domain pairs, whereas 90° domain walls facilitate transitions between these pairs.

The study also observed vortex topological structures formed at the junction of four different ferroelectric domain orientations, which exhibit exceptional topological stability.

This study marks the first time that topological domain structures dominated by charged domain walls, and their dynamic evolution, have been revealed in an elemental 2D ferroelectric, providing a new theoretical foundation and pathway for designing novel ferroelectric topological materials, with potential applications in high-density data storage, neuromorphic computing, quantum devices, and flexible electronics.

The research has been published in the prestigious physics journal Physical Review Letters under the title Domain-Pair Intertwined Topological Domain Structures in Elemental Bi Monolayer.