A Polar Topology Network Induces Elastic Stiffening in Ferroelectric Oxide Superlattices.

In oxide superlattices, the combined effect of strain and interlayer electrostatic interactions enables the stabilization of exotic polar topologies. Yet, their collective role in the mechanical response of the system remains largely unexplored. Here, we show that a high density of polar textures forms an elastically constrained network that stiffens the heterostructure. In our model system of BiFeO3/SrTiO3 superlattices, systematic variation of periodicity tunes the balance between the depolarization field and interlayer coupling, driving a deterministic transition from smeared polar nanodomains to a topology-rich regime and then to a labyrinthine multidomain state. Transmission electron microscopy reveals that this topological evolution is accompanied by pronounced strain heterogeneity in the BiFeO3 layer and an induced tetragonality in the dielectric spacer, suggesting a more active role played by the dielectric layer in the topological phase evolution. Remarkably, nanoindentation experiments show that the topology-rich configuration exhibits a prominent 20% increase in the effective elastic modulus compared to the multidomain state. Further, Monte Carlo simulations show how the topology governs mechanical response through topological defect-induced stiffening. These results establish a jammed-like scenario in which dense topological defects collectively restrict stress accommodation pathways and highlight periodicity as a design parameter for topology-mediated response in polar oxide superlattices.