Squishing Skyrmions: Symmetry-Guided Dynamic Transformation of Polar Topologies Under Compression

Abstract

Mechanical controllability of recently discovered topological defects (e.g., skyrmions) in ferroelectric materials is of interest for the development of ultralow-power mechano-electronics that are protected against thermal noise. However, fundamental understanding is hindered by the “multiscale quantum challenge” to describe topological switching encompassing large spatiotemporal scales with quantum mechanical accuracy. Here, we overcome this challenge by developing a machine-learning-based multiscale simulation frameworka hybrid neural network quantum molecular dynamics (NNQMD) and molecular mechanics (MM) method. For nanostructures composed of SrTiO3 and PbTiO3, we find how the symmetry of mechanical loading essentially controls polar topological switching. We find under symmetry-breaking uniaxial compression a squishing-to-annihilation pathway versus formation of a topological composite named skyrmionium under symmetry-preserving isotropic compression. The distinct pathways are explained in terms of the underlying materials’ elasticity and symmetry, as well as the Landau–Lifshitz–Kittel scaling law. Such rational control of ferroelectric topologies will likely facilitate exploration of the rich ferroelectric “topotronics” design space.

Induction and Ferroelectric Switching of Flux Closure Domains in Strained PbTiO3 with Neural Network Quantum Molecular Dynamics

“…13−15 For example, Linker et al have developed multiscale NNQMD simulation frame-works to explore optical and mechanical manipulation of various topological structures in PbTiO 3 (PTO) based ferroelectric structures. 8,12 However, studying the electric field dynamics with ML interatomic potentials poses a unique challenge, as there is no description of atomic charge in commonly available NNQMD models. Notable breakthrough are so-called the third and fourth generation high-dimensional neural network potentials (HDNNPs) that incorporate some form of charge/coulomb interaction and dynamic charge transfer.…”

“…We examine the dynamics of ferroelectric switching of bulk PbTiO 3 utilizing the existing NNQMD force field developed in ref 12. The previously developed NNQMD model was used to predict the ground-state forces, whereas the Born effective charge tensor at the equilibrium PbTiO 3 position from the literature 25 was used to apply the electric field.…”

“…Over past few years, there has been considerable development in forming topological structures (e.g., skyrmions, merons, vortices, and flux closures) in strained oxide superlattices. − Dynamical control of these topological structures has garnered great interest for the development of next-generational ferroelectric devices, as they are topologically protected from thermal noise and can be directly manipulated through control of the mechanical strain and polarization boundary conditions that give rise to their emergence. This has created a new field of “ferroelectric topotronics”, where one tries to manipulate topological defects through optical, mechanical, or electrical means. − …”

“…Here, we propose a simple extension based on the BEC method combined with the existing NNQMD model , to investigate switching dynamics in an archetypal PTO system. Our NNQMD-BEC scheme naturally takes into account the rescreening effect by the BEC extension to first order.…”

“…The model is trained in the Aenet software and implemented in the RXMD molecular dynamics software . Complete description of the training process is provided in ref . The used NNQMD model was previously validated using various elastic and structural properties and also examined the dynamics of 180° DWs and polar skyrmion domains that has shown consistent behavior with state-of-the-art experimental imaging techniques and phase-field simulation studies. , The system consists of 40 × 40 × 40 PTO unit cells with uniform polarization along the negative c -axis at the beginning of the simulation.…”