Emergent Dipolar Textures and Phenomena in Ferroelectric Superlattices

Emergent topological dipolar textures including vortices, skyrmions, merons, hopfions, etc. have generated considerable interest in the ferroelectrics and condensed-matter physics communities. Balancing the electric, elastic, and gradient energies of ferroelectrics in low-dimensional forms allows us to manipulate the order parameter in new ways and elicit exotic function. Ferroelectric-dielectric superlattices [e.g., (PbTiO₃)_n/(SrTiO₃)_n] have served as the model systems for this work thus far. Here, we discuss some of the latest observations in these materials. First, we explore the evolution of these emergent textures with temperature where initial data has suggested broad hysteresis and logarithmic changes in correlation lengths unlike transitions seen in the parent materials. Studying the materials from 70-750 K with X-ray diffraction and dielectric measurements, we identify the paraelectric-to-ferroelectric transition, a transition into a coexistence of periodic nano-domains and vortex structures, and a subsequent transition slowdown akin to glassy behavior. We will explore this “melting-like” transition which is reminiscent of a BKT transition. From there, we transition into discussions of all-ferroelectric superlattices, of the form Pb_1-xSr_xTiO₃/PbTiO₃ wherein 0.5 < x < 1. We will explore the potential to produce both polar vortices and polar skyrmions in these heterostructures which are considerable more tunable due to all layers being ferroelectric. These all-ferroelectric heterostructures offer dramatically improve field evolution including the potential to reversibly drive the system through a topological-phase transition from the a nontrivial-skyrmion to a trivial-ferroelectric state. We will also explore the memory/history of these transitions and their impacts on macroscopic properties such as piezoelectricity and ferroelectric switching.