Lei Yunlin, Yang Xinyu, Wang Shouyu, Zhang Daliang, Wang Zitao, Zhang Jiayou, Yang Yihao, Wang Chuanshou, Xiao Tianqi, Bai Yinxin, Tian Junjiang, Chen Congcong, Han Yu, Dong Shuai, Wang Junling
Department of Physics & Guangdong Provincial Key Laboratory of Functional Oxide Materials and Devices, Southern University of Science and Technology, Shenzhen 518055, China.
Key Laboratory of Quantum Materials and Devices of Ministry of Education, School of Physics, Southeast University, Nanjing 211189, China.
Natl Sci Rev. 2025 Aug 8;12(9):nwaf320. doi: 10.1093/nsr/nwaf320. eCollection 2025 Sep.
Materials possessing long-range ordering of magnetic spins or electric dipoles have been the focus of condensed matter research. Among them, ferri-systems with two sublattices of unequal/non-collinear spins or electric dipoles are expected to combine the properties of ferro- and antiferro-systems, but lack experimental observations in single-phase materials. This is particularly true for the ferrielectric system, since the electric dipoles can usually be redefined to incorporate the two sublattices into one, making it indistinguishable from ferroelectric. This raises doubts about whether or not ferrielectricity can be considered as an independent ferroic order. Here we report the observation of true ferrielectric behaviors in a hybrid single crystal (MV)[SbBr] (MV = ,'-dimethyl-4,4'-bipyridinium or methyl viologen), where the two electric dipole sublattices switch asynchronously, and thus cannot be reduced to ferroelectric by redefining the unit cell. Furthermore, the complex dipole configuration imparts circularly polarized light sensitivity to the system. An electric field can modulate the non-collinear dipole sublattices and even induce a transition from ferrielectric to ferroelectric state, thereby tuning the helicity-dependent photocurrent. This study opens a new paradigm for the study of true irreducible ferrielectricity (a new class of polar systems) and provides an effective approach to the electric field control of spin-orbit coupling and circular photogalvanic effect.
具有磁自旋或电偶极子长程有序的材料一直是凝聚态物质研究的重点。其中，具有不等/非共线自旋或电偶极子的两个亚晶格的铁电体系有望兼具铁电和反铁电体系的性质，但在单相材料中缺乏实验观测。对于铁电介质体系尤其如此，因为电偶极子通常可以重新定义，将两个亚晶格合并为一个，使其与铁电体难以区分。这引发了关于铁电介质是否可被视为一种独立的铁性序的疑问。在此，我们报告了在混合单晶（MV）[SbBr]（MV = 1,1'-二甲基-4,4'-联吡啶鎓或甲基紫精）中观察到的真正的铁电介质行为，其中两个电偶极子亚晶格异步切换，因此不能通过重新定义晶胞而归结为铁电体。此外，复杂的偶极子构型赋予该体系圆偏振光敏感性。电场可以调制非共线偶极子亚晶格，甚至诱导从铁电介质到铁电状态的转变，从而调节与螺旋度相关的光电流。这项研究为真正不可约的铁电介质（一类新的极性体系）的研究开辟了新范式，并为自旋轨道耦合和圆光电流效应的电场控制提供了有效方法。
