Ferroelectric M B i2 T e4 (M=Ge, Sn, Pb) bilayers with giant Rashba spin splitting and tunable first- or second-order topological states

Ferroelectricity, the Rashba effect, and band topology have been individually explored in various two-dimensional (2D) materials. Investigating their mutual couplings offers exciting opportunities for understanding complex physics and unlocking new functionalities. However, the coexistence and modulation of all three properties within a single 2D system remain rare. In this work, we unveil a mechanism to achieve this synergy through interlayer sliding in 2D MBi2Te4 (M = Ge, Sn, Pb) bilayers, in which a centrosymmetric MBi2Te4 monolayer intrinsically manifests as a second-order topological insulator. Our findings reveal that bilayer MBi2Te4 exhibits ferroelectricity combined with giant Rashba spin splitting (with αR of ∼2.2 eV Å) and protected one-dimensional edge states under distinctive and robust stacking orders. As well, interlayer sliding enables modulation of both ferroelectric polarization and Rashba spin textures and facilitates transitions between topological insulator and second-order topological insulator phases. Our results provide pathways for the innovation of next-generation multifunctional electronic nanodevices with the ability to combine ferroelectric, spintronic, and topotronic technologies.