Emergence of complex polar topologies in oxide heterostructures

Complex topological configurations are a fertile arena to explore novel emergent phenomena and exotic phases in condensed-matter physics. The recent discovery of polarization vortices and the associated complex-phase coexistence and response under applied fields in ferroelectric oxide superlattices, has opened up new vistas to explore topology, emergent phenomena, and approaches for manipulating such features with electric fields1,2. Here, by varying epitaxial constraints we report the discovery of room-temperature polar skyrmions in a lead-titanate layer confined by strontium-titanate layers, imaged by atomic-resolution scanning transmission electron microscopy3. Phase-field modeling and second-principles calculations reveal that the polar skyrmions have a skyrmion number of +1, and resonant soft X-ray diffraction experiments show circular dichroism confirming chirality. Such nanometer-scale polar skyrmions are the electric analogs of magnetic skyrmions, and could advance ferroelectrics towards new levels of functionality. Using macroscopic dielectric measurements, we demonstrate that polar skyrmions in (PbTiO3)n/(SrTiO3)n superlattices are distinguished by a sheath of negative permittivity at the periphery of each skyrmion which enables a strong enhancement of the effective dielectric permittivity as compared to the individual SrTiO3 and PbTiO3 layers4 and phenomenon could be controlled by electric field and temperature. Such phenomena could advance ferroelectrics towards new levels of functionality.

1	Yadav, A.K., Nelson, C.T. et al., Observation of polar vortices in oxide superlattices. Nature530, 198-201 (2016).
2	Damodaran, A., Clarkson, J., Hong, Z., Liu, H. et al., Phase coexistence and electric-field control of toroidal order in oxide superlattices. Nat. Mater.16, 1003 (2017).
3	Das, S et al, Observation of room temperature polar skyrmions. Nature 568, 368-372 (2019).
4	Yadav, A. K. et al, Spatially Resolved Steady State Negative Capacitance. Nature 565, 468-471 (2019).