Edge Polarization Topology Integrated with Sliding Ferroelectricity in Moiré System

Van der Waals moiré heterostructure have been found to exhibits a robust interfacial ferroelectricity down to atomic thickness, and discovering and understanding the complex polarization state in moiré systems is of fundamental interest to condensed-matter research. In this study, we examined the moiré ferroelectricity in twisted h-BN heterostructure by piezoresponse force microscopy. Due to atomic reconstruction, triangular moiré patterns are detected, and we have directly observed sliding ferroelectricity in the center of triangular moiré patterns as well as robust in-plane polarization topology emerging at the boundary of adjacent triangles, which we call edge ferroelectricity. The edge ferroelectricity possesses non-trivial vortex polarization topology. Our calculations trace the origin of this phenomenon to joined piezoelectric effects with sliding ferroelectricity. The present work provides intuitive insights to explore the unique moiré ferroelectricity in a non-polar background matrix, paving the way for potential ultrathin nonvolatile memory applications.