Phase coexistence and electric-field control of toroidal order in oxide superlattices

Abstract

Systems that exhibit phase competition, order parameter coexistence, and emergent order parameter topologies constitute a major part of modern condensed-matter physics. Here, by applying a range of characterization techniques, and simulations, we observe that in PbTiO/SrTiO superlattices all of these effects can be found. By exploring superlattice period-, temperature- and field-dependent evolution of these structures, we observe several new features. First, it is possible to engineer phase coexistence mediated by a first-order phase transition between an emergent, low-temperature vortex phase with electric toroidal order and a high-temperature ferroelectric a/a phase. At room temperature, the coexisting vortex and ferroelectric phases form a mesoscale, fibre-textured hierarchical superstructure. The vortex phase possesses an axial polarization, set by the net polarization of the surrounding ferroelectric domains, such that it possesses a multi-order-parameter state and belongs to a class of gyrotropic electrotoroidal compounds. Finally, application of electric fields to this mixed-phase system permits interconversion between the vortex and the ferroelectric phases concomitant with order-of-magnitude changes in piezoelectric and nonlinear optical responses. Our findings suggest new cross-coupled functionalities.

Deterministic Switching of Ferroelectric Bubble Nanodomains

“…Topological structures (or defects) in ferroic materials, such as skyrmions, vortices, flux closures, incommensurate curl domains, and bubble domains, have recently attracted attention due to their emergent nature. They can display large current‐induced spin‐orbit torques, giant electromechanical response, enhanced or ballistic electron transport, chirality or colossal optical coefficients, to name just a few of their exciting properties. Often these are distinctly absent in their parent ferroic phases, which provides us with a new paradigm for functional nanodevices …”

“…These have been demonstrated in magnetic systems, e.g., the use of magnetic skyrmions as well as magnetic domain‐walls for race‐track memory. However, controlled switching at the single defect level in ferroelectrics remains a substantive challenge2c,10,14 even though this has been achieved at the mesoscale . The unambiguous demonstration of deterministic switching between various nontrivial ferroelectric topologies, particularly at the single defect level would lead to a unprecedented handle over the emergent properties and hence entirely new cross‐coupled electro‐mechanical‐optical‐polarization functionalities, as outlined by Martin et al The primary impediment with ferroelectrics is that single topological configurations (i.e., vortex‐like or bubble domain states) have a size only several unit‐cells, which is several orders magnitude smaller than their classical magnetic counterparts.…”

Interface Engineering and Emergent Phenomena in Oxide Heterostructures

“…Although the enhancement of the ferroelectric response at the oxide interface is possible, the major challenge is the depolarization field and the suppression on ferroelectricity at the interface is often observed . Recently, the coexistence of a ferroelectric phase and vortex phase with an electric toroidal order has been demonstrated at the PbTiO 3 /SrTiO 3 superlattice, where the applied electric field can trigger the conversion between those two phases causing large changes in the piezoelectric response . On the other hand, by taking advantage of polar nanoregions (PNRs), which are usually induced by chemical defects or structural disorders, the ferroelectricity can be enhanced when reducing the film thickness or dimension .…”

Probing Ferroic States in Oxide Thin Films Using Optical Second Harmonic Generation

“…Regarding thin films, the optical investigation of domain walls remains challenging due to the optical resolution limit and the limited SHG active domain wall volume. Recent progress dealing with laser-scanning SHG microscopy [128] and near-field SHG nano-imaging [129,130] where tip-enhanced SHG signals are collected with a spatial resolution approaching the submicron range [131,132] has been achieved and opens new avenues towards the symmetry analysis of nanoscale objects.…”