Emergent helical texture of electric dipoles

TL;DR

This paper reports the discovery of incommensurate helical ordering of electric dipoles in a doped dielectric crystal, revealing a new type of long-range electric dipole structure stabilized by competing instabilities.

Contribution

It introduces the observation of electric dipole helicoidal textures in a dielectric crystal, a phenomenon previously known only in magnetic systems.

Findings

Incommensurate electric dipole helical order observed in BiMn7O12

Phase transitions from cubic to helical structures via density wave

Competition between orbital ordering and lone electron activity stabilizes the helix

Abstract

Long-range ordering of magnetic dipoles in bulk materials gives rise to a broad range of magnetic structures, from simple collinear ferromagnets and antiferromagnets, to complex magnetic helicoidal textures stabilized by competing exchange interactions. In contrast, in the context of dipolar order in dielectric crystals, only parallel (ferroelectric) and antiparallel (antiferroelectric) collinear alignments of electric dipoles are typically considered. Here, we report an observation of incommensurate helical ordering of electric dipoles by light hole-doping of the quadruple perovskite BiMn7O12. In analogy with magnetism, the electric dipole helicoidal texture is also stabilized by competing instabilities. Specifically, orbital ordering and lone electron pair stereochemical activity compete, giving rise to phase transitions from a non-chiral cubic structure, to an incommensurate electric dipole and orbital helix, via an intermediate density wave.