Noninvasive Three-dimensional Mapping of Polar Skyrmion Structures with Atomic Resolution

Functional oxide materials may exhibit intricate interactions between spin, orbital, and lattice order parameters and with the signifcant advances in thin film deposition techniques, it is now possible to engineer interfaces with unit-cell precision to induce novel emergent phenomena such as chirality and negative capacitance.
The stabilization of polar skyrmions in superlattices consisting of short-period ferroelectric lead titanate and dielectric strontium titanate are prominent examples of most advanced capacity in electric dipole textures engineering. To date, however, experimental evidence of polar skyrmion ordering has primarily relied on invasive or destructive imaging techniques or microstructure X-ray analysis without atomic resolution. A non-invasive investigation of these domains that is representative of the whole sample and provides atomic resolution remains challenging, but promises deeper insights into their structure and distribution.
In this work, we present a new non-invasive method for three-dimensional mapping of polar skyrmion domains through large-volume reciprocal space investigation. Specifically, in our skyrmions hosting lead titanate / strontium titanate superlattices, we acquired extensive three-dimensional diffuse scattering data using synchrotron hard X-rays in an ultra-small grazing-incidence geometry. The data was analyzed with three-dimensional delta pair distribution function (3D-ΔPDF) method, which enabled us to extract detailed information about the atomic order in the superlattice domains including 3D distribution as well as interlayer correlations of the domains. This experimental strategy paves the way for real structure analysis of thin films, offering insights not esily obtainable through conventional techniques such as Piezoresponse Force Microscopy (PFM) or Scanning Transmission Electron Microscopy (STEM).