Controllable topological phases driven by polarizations and magnetic fields in ferroelectric-based heterobilayers

Achieving precise control of the topological phases spanning normal insulators, topological insulators (TIs), and second-order TIs (SOTIs) is still rare in two-dimensional atomic crystals. Herein, through a low-energy effective model, we propose a generalized design strategy that enables the systematic control of nontrivial topology in heterobilayers under ferroelectric polarization reversal and an external magnetic field. Our analysis reveals that the polarized state not only induces the band inversion to form a TI phase but also breaks its out-of-plane mirror symmetry, thereby enabling the realization of SOTIs under weak magnetic fields. Moreover, first-principles calculations and topological analysis have confirmed that a series of heterobilayers exhibit switchable topological states manifested through quantized spin Hall conductance, edge states, and spatially resolved zero-dimensional corner states. This work not only provides a practical framework for engineering a normal insulator to achieve the TI/SOTI phase but also opens up a technological avenue bridging first-order and higher-order band topologies.