Ultrasmall Polar Skyrmions and Merons in SrTiO3 Heterostructures by Polaron Engineering

Topological objects with skyrmionic textures in ferroelectrics,i.e., polar skyrmions, are promising technological paradigms in next-generationelectronic devices. While breakthrough discoveries of stable polarskyrmions approximately ten nanometers in size have been very recentlywitnessed in complex systems, such a nontrivial topological orderin ferroelectrics inevitably disappears below the ferroelectric criticalsize of several nanometers. Herein, we propose a strategy to overcomethis limitation and achieve ultrasmall and isolated polar skyrmionsby engineering excess-electron polarons in otherwise nonferroelectricSrTiO(3) heterostructures. Our first-principle calculationsdemonstrate that a polaron localized at a SrTiO3 surfaceinduces a Neel-type polar skyrmion as small as 1.8 nanometers attributedto the effect of atomic-scale surface roughness. Furthermore, we showthat this polar topological structure is tunable by the choice ofheterostructures and by the mechanical approach, which undergoes aphase transition to a meron state in the twisted boundary and to anantiskyrmion state in the surface with external shear strain, respectively.Such ultraminiaturization of skyrmions and their transitions unexpectedlyunravels the formula of ultrasmall topological orders originatingfrom the interplay between an electron polaron and structural symmetrybreaking, which is completely different from the common mechanismof geometric confinement for larger-scale skyrmions. Our results notonly provide a mechanism for the exploration of polar skyrmions andtheir rich topological transitions but also hold potential for ultrahigh-densitymemories.