Dipolar wavevector interference induces a polar skyrmion lattice in strained BiFeO films.

Skyrmions can form regular arrangements, so-called skyrmion crystals (SkXs). A mode with multiple wavevectors q then describes the arrangement. While magnetic SkXs, which can emerge in the presence of Dzyaloshinskii-Moriya interaction, are well established, polar skyrmion lattices are still elusive. Here we report the observation of polar SkXs with a well-defined double-q state in ultrathin BiFeO films on LaAlO. The compressive strain induced by the LaAlO substrate yields a dipolar topological texture with a periodic arrangement of skyrmions. The square-like superstructure with a lattice constant of 2.68 nm features a periodic modulation of polarization fields and topological charge density. The film furthermore exhibits an enhanced electromechanical response with an increased converse piezoelectric coefficient (d) compared with SkX-free films. Transmission electron microscopy experiments in combination with phase-field simulations indicate that the dipole skyrmion texture results from the interference of two orthogonal single-q dipole patterns. We anticipate that the interference of multiple wavevectors may lead to a diversity of topological crystals with a variety of symmetries and lattice constants.