Polar vortex hidden in twisted bilayers of paraelectric SrTiO3

Abstract: Polar topologies, such as vortex and skyrmion, have attracted significant interest due to their unique physical properties and promising applications in high-density memory devices. Currently, most polar vortices are observed in heterostructures containing ferroelectric materials and constrained by substrates. In this study, we unravel arrays of polar vortices formed in twisted freestanding bilayers composed of SrTiO3, a quantum-paraelectric material. Depth-resolved structures of the bilayers are measured with deep-sub-angstrom resolution and one picometer accuracy using multislice ptychography, enabling identification of the three-dimensional variations of polarization topology. Our findings reveal the evolution of the polar vortices in the twisted overlapping layers, demonstrating the reverse of rotation manner in the depth direction. Twisted freestanding bilayers provide a unique platform for exploration and modulation of novel polar topologies.