Emergent helical texture of electric dipoles

Long-range ordering of magnetic dipoles in bulk materials gives rise to a broad range of magnetic structures, from simple collinear ferromagnets to complex magnetic helicoidal textures stabilized by competing exchange interactions. In contrast, dipolar order in dielectric crystals is typically limited to parallel (ferroelectric) and antiparallel (antiferroelectric) alignments of electric dipoles. In this talk I will report our recent discovery of an emergent, incommensurate helical ordering of electric dipoles by light hole doping of the quadruple perovskite BiMn₇O₁₂. I will show, in close analogy with magnetism, that the electric dipole helicoidal texture is stabilized by competing structural instabilities. Specifically, orbital ordering and lone electron pair stereochemical activity compete, giving rise to phase transitions from a nonchiral cubic structure to an incommensurate electric dipole and orbital helix via an intermediate dipole density wave. This new ground state also supports weak improper ferroelectricity, implying that an electric field might be used to switch the chirality of the electric dipole helix. This remarkable functionality could be utilised, for example, in an electric-field-controllable, optically active device.