Sliding Ferroelectrics Induced Hybrid-Order Topological Phase Transitions

We propose ferroelectric layer sliding as a new approach to realize and manipulate topological quantum states in two-dimensional (2D) bilayer magnetic van der Waals materials. We show that stacking monolayer ferromagnetic topological states into layer-spin-locked bilayer antiferromagnetic structures, and introducing sliding ferroelectricity leads to asynchronous topological evolution of different layers (spins) owing to the existence of polarization potentials, thereby giving rise to rich layer-resolved topological phases. As a specific example, by means of a lattice model, we show that a bilayer magnetic 2D second order topological insulator (SOTI) reveals an unrecognized spin-hybrid-order topological insulator after undergoing ferroelectric sliding. Interestingly, in such a phase, the spin-up (top layer) and spin-down (bottom layer) channels exhibit first-order and second-order topological properties, respectively. Moreover, other topological phases such as the SOTI, quantum spin Hall insulator, quantum anomalous Hall insulator, and trivial insulator, can also emerge through changes in the parameters of the system; and the relevant topological indices are also discussed. In terms of materials, based on first principles calculations, we predict the material ScI2 can serve as an ideal platform to realize our proposal. Further, we predict that the anomalous Nernst effect of these several topological phases exhibits distinct differences, and therefore can be used as a signal for experimental probing.