Abstract
A new type of nanoscale ferroelectric domains, termed as “bubble domains”, has been observed in ultrathin epitaxial PbZr0.2Ti0.8O3/SrTiO3/PbZr0.2Ti0.8O3 ferroelectric sandwich structures. The formation of bubble domains are attributed to this particular sandwich heterostructure which results in the competition between incomplete polarization screening (electrostatic) and compressive mechanical strain (elastic) parameters. The existence of the bubble domains is revealed by high-resolution piezoresponse force microscopy (PFM), and is corroborated by aberration-corrected atomic-resolution scanning transmission electron microscopy mapping of the polarization displacements.  Using above techniques, it is confirmed that the bubble domain are laterally confined spheroids of sub-10 nm size with local dipoles opposite to the macroscopic polarization of their surrounding ferroelectric matrix. An incommensurate phase and symmetry breaking is found within these domains, which result in local polarization rotation and hence impart a mixed Néel-Bloch-like character to the bubble domain walls. PFM hysteresis loops for the bubble domains reveal that they undergo an irreversible phase transition to cylindrical domains under the electric field, accompanied by a transient rise in the electromechanical response. Our observations are in agreement with ab-initio-based calculations, which reveal a very narrow window of electrical and elastic parameters that allow the existence of bubble domains. The findings highlight the richness of polar topologies that may develop in ultrathin ferroelectric structures and bring forward the prospect of emergent electronic functionalities due to topological transitions.

CV:
Dr Qi Zhang received her bachelor and master degrees from Huazhong University of Science and Technology, and finished her PhD in University of New South Wales in 2015. She is working as a Research Associate (Postdoc fellow) in UNSW and her research interests include piezoresponse force microscopy, topological domain structures in ferroelectric thin films, nano-device fabrication, and chemical solution deposition.