Probing Topological Phase Transitions in Real-Space Polar Textures

Real-space topological dipolar textures such as polar vortices, skyrmions and merons in ferroelectric heterostructures emerge resulting from the interplay of elastic, electrostatic and gradient energies. The emergence of these dipolar textures is often accompanied by functionalities such as emergent chirality and local negative capacitance for potential applications in next generation nanodevices. As these textures are intrinsically three-dimensional (3D), nm-sized objects, it poses a challenge for characterizing their detailed atomic structures as well as to understand and explore their potential topological phase transitions.<br/><br/>Here, we explore the topological phase transitions in epitaxial perovskite oxide heterostructures, for example in systems of (PbTiO₃)/(SrTiO₃) and (BiFeO₃)/(TbScO₃), using a combination of atomic resolution imaging, and four-dimensional scanning transmission electron microscopy (4D-STEM). Using 4D-STEM multislice ptychography, we found a new phase in multiferroic BiFeO₃, the 3D dipolar waves, which can be characterized by incommensurate periodicities and antiferrodistortive modes. Further, 4D-STEM-based approach enabled us to directly observe the transitions among polar skyrmions, merons and anti-merons, with some transitions accompanied by a change in their chirality.<br/><br/>Research supported by AFOSR Hybrid Materials MURI (FA9550-18-1-0480), ARO ETHOS MURI (W911NF-21-2-0162), and USC Viterbi start-up funds. Facilities support from the NSF (DMR-1429155, DMR-1719875, DMR-2039380). Researchers at the University of Arkansas also thank the Vannevar Bush Faculty Fellowship Grant No. N00014-20-1-2834 from the Department of Defense.