ISIS Facility, Rutherford Appleton Laboratory, Harwell Campus, Didcot OX11 0QX, UK.
Department of Physics and Astronomy, University College London, London WC1E 6BT, UK.
Science. 2020 Aug 7;369(6504):680-684. doi: 10.1126/science.aay7356.
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, dipolar order in dielectric crystals is typically limited to parallel (ferroelectric) and antiparallel (antiferroelectric) collinear alignments of electric dipoles. Here, we report an observation of incommensurate helical ordering of electric dipoles by light hole doping of the quadruple perovskite BiMnO In analogy with magnetism, the electric dipole helicoidal texture is stabilized by competing 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 density wave.
在体材料中，磁偶极子的长程有序导致了广泛的磁结构，从简单的共线铁磁体和反铁磁体，到由竞争交换相互作用稳定的复杂螺旋磁织构。相比之下，介电晶体中的偶极有序通常仅限于电偶极子的平行（铁电）和反平行（反铁电）共线排列。在这里，我们通过四重钙钛矿 BiMnO 的光孔掺杂报告了电偶极子的不均匀螺旋有序的观察结果。与磁性类似，电偶极子螺旋织构通过竞争不稳定性来稳定。具体来说，轨道有序和孤电子对立体化学活性竞争，导致从非手性立方结构到不均匀电偶极子和轨道螺旋的相变，通过中间密度波。
