Unified structural origin of functionality in spiral skyrmions revealed by 3D electron tomography

Abstract Ferroelectric skyrmions exhibit a range of emergent phenomena, including collective dynamics, negative capacitance, and a strong electric-field-induced second-harmonic generation response. However, three-dimensional (3D) atomic-scale configurations of skyrmions are unclarified yet, making the relationship between the physical functionalities and their underlying structural origin remains elusive. Here, using atomic-scale electron tomography, we experimentally resolve the 3D structure of polar skyrmion for the first time in PbTiO3/SrTiO3 superlattice. On the basis of large-data image acquisition with more than 95,000 3D coordinates of atoms, we visualize that the core of the skyrmions exhibit an upward spiraling morphology. By quantitatively analyzing the ferroelectric polarization and lattice distortion, we uncover an unconventional decoupling between the two at the skyrmion wall, where the polarization is suppressed, accompanied by a local enhancement of tetragonality exceeding 5% at the center of the wall. This increased tetragonality modifies the local free-energy landscape, enhancing the negative-capacitance response and field sensitivity at the skyrmion wall. The polar skyrmion wall hosts a concentration of emergent physical properties, thereby providing a unified structural origin for the functionalities and physical behaviors of skyrmions. This work establishes a real space basis for understanding the physical properties of polar skyrmions.