Shimada Takahiro, Wang Yu, Hamaguchi Takayuki, Kasai Kohta, Masuda Kairi, Van Lich Le, Xu Tao, Wang Jie, Hirakata Hiroyuki
Department of Mechanical Engineering and Science, Kyoto University, Nishikyo-ku, Kyoto 615-8540, Japan.
School of Materials Science and Engineering, Hanoi University of Science and Technology, No 1, Dai Co Viet Street, Hanoi 100000, Vietnam.
J Phys Condens Matter. 2021 Oct 5;33(50). doi: 10.1088/1361-648X/ac28c1.
Discovery of non-trivial topological structures in condensed matters holds promise in novel technological paradigms. In contrast to ferromagnetics, where a variety of topological structures such as vortex, meron, and skyrmion have been discovered, only few topological structures can exist in ferroelectrics due to the lack of non-collinear interaction like the Dzyaloshinskii-Moriya interaction in ferromagnetics. Here, we demonstrate that polarization structures with a wide range of topological numbers (winding numberfrom -3 to +1) can be mechanically excited and designed by the mode-I singular stress field formed near the crack-tip in incipient ferroelectric SrTiO. Our phase-field simulations based on Ginzburg-Landau theory successfully reveals that the near-tip polar topology is driven by the flexoelectric coupling with intense strain gradient at the tip, while a variety of the far-field topological structures is triggered by a collaboration between the electrostrictive and flexoelectric effects. The strain (gradient) field analysis further shows that the unexpected topological characters are implied in the singular stress field, which develops a variety of polar topologies near the crack tip. Therefore, our work provides a novel insight into the unusual interplay between mechanical- and ferroelectric-topologies, i.e. 'topological strain-field engineering', which paves the way to the mechanical design of functional topologies in the matter.
在凝聚态物质中发现非平凡拓扑结构有望带来新的技术范式。与铁磁体不同，在铁磁体中已发现了多种拓扑结构，如涡旋、磁单极子和斯格明子，而由于缺乏像铁磁体中存在的Dzyaloshinskii-Moriya相互作用那样的非共线相互作用，铁电体中只能存在少数拓扑结构。在此，我们证明了具有广泛拓扑数（缠绕数从 -3 到 +1）的极化结构可以通过在初始铁电体SrTiO中裂纹尖端附近形成的I型奇异应力场进行机械激发和设计。我们基于金兹堡 - 朗道理论的相场模拟成功揭示，尖端附近的极性拓扑由尖端处具有强烈应变梯度的挠电耦合驱动，而各种远场拓扑结构则由电致伸缩和挠电效应之间的协同作用触发。应变（梯度）场分析进一步表明，奇异应力场中隐含着意想不到的拓扑特征，该应力场在裂纹尖端附近产生了各种极性拓扑。因此，我们的工作为机械拓扑和铁电拓扑之间的异常相互作用，即“拓扑应变场工程”提供了新的见解，为物质中功能拓扑的机械设计铺平了道路。
