Emergent chirality in the electric polarization texture of titanate superlattices

Significance Many natural structures exhibit chirality that is essential to their functional interactions, yet the chiral electronic structures found in condensed matter systems have been primarily limited to magnetic materials. Notably, the electric dipole equivalent of magnetic skyrmions has remained conspicuously elusive. However, recent theoretical predictions and experimental observations of the continuous rotation of electric polarization in titanate superlattices suggests that such complex oxide nanocomposites are ideal candidates for realizing chiral electric dipole configurations. Here we present the results from superlattices of PbTiO3 and SrTiO3 using a combination of resonant soft X-ray diffraction and second-principles simulations. We observe chiral arrays of polar line defects, spontaneously formed by the complex interactions in these artificial superlattices constructed from two nonchiral lattices. Chirality is a geometrical property by which an object is not superimposable onto its mirror image, thereby imparting a handedness. Chirality determines many important properties in nature—from the strength of the weak interactions according to the electroweak theory in particle physics to the binding of enzymes with naturally occurring amino acids or sugars, reactions that are fundamental for life. In condensed matter physics, the prediction of topologically protected magnetic skyrmions and related spin textures in chiral magnets has stimulated significant research. If the magnetic dipoles were replaced by their electrical counterparts, then electrically controllable chiral devices could be designed. Complex oxide BaTiO3/SrTiO3 nanocomposites and PbTiO3/SrTiO3 superlattices are perfect candidates, since “polar vortices,” in which a continuous rotation of ferroelectric polarization spontaneously forms, have been recently discovered. Using resonant soft X-ray diffraction, we report the observation of a strong circular dichroism from the interaction between circularly polarized light and the chiral electric polarization texture that emerges in PbTiO3/SrTiO3 superlattices. This hallmark of chirality is explained by a helical rotation of electric polarization that second-principles simulations predict to reside within complex 3D polarization textures comprising ordered topological line defects. The handedness of the texture can be topologically characterized by the sign of the helicity number of the chiral line defects. This coupling between the optical and novel polar properties could be exploited to encode chiral signatures into photon or electron beams for information processing.