Terahertz-field activation of polar skyrons

TL;DR

This paper reports the discovery of 'skyrons', novel collective modes in ferroelectric superlattices activated by terahertz fields, revealing new topological dynamics and control possibilities at the atomic scale.

Contribution

It introduces the concept of skyrons, a new type of collective mode in polar skyrmions, and demonstrates their control via terahertz excitation and external parameters.

Findings

Discovery of subterahertz skyrons with swirling atomic displacement patterns.

Observation of avoided crossing in skyrons' dispersion relation.

Control of skyrons' amplitude and dispersion through temperature and electric bias.

Abstract

Unraveling collective modes arising from coupled degrees of freedom is crucial for understanding complex interactions in solids and developing new functionalities. Unique collective behaviors emerge when two degrees of freedom, ordered on distinct length scales, interact. Polar skyrmions, three-dimensional electric polarization textures in ferroelectric superlattices, disrupt the lattice continuity at the nanometer scale with nontrivial topology, leading to previously unexplored collective modes. Here, using terahertz-field excitation and femtosecond x-ray diffraction, we discovered subterahertz collective modes, dubbed 'skyrons', which appear as swirling patterns of atomic displacements functioning as atomic-scale gearsets. Momentum-resolved time-domain measurements of diffuse scattering revealed an avoided crossing in the dispersion relation of skyrons. We further demonstrated that the amplitude and dispersion of skyrons can be controlled by sample temperature and electric-field bias. Atomistic simulations and dynamical phase-field modeling provided microscopic insights into the three-dimensional crystallographic and polarization dynamics. The discovery of skyrons and their coupling with terahertz fields opens avenues for ultrafast control of topological polar structures.