二维IV型磁体MgCr$_2$O$_4$中的高阶拓扑相

中文摘要 AI 辅助

IV型二维（2D）磁性是一类新划分的共线磁相，具有非相对论自旋简并和自旋轨道耦合诱导的动量依赖自旋劈裂特性，拓展了共线磁体的对称性分类，为非常规拓扑量子态开辟了新机遇。本研究发现，近期提出的二维IV型磁体MgCr$_2$O$_4$存在本征高阶拓扑绝缘相，该相具有受$\u27e8$C$_{3z}$$\u27e9$保护的角态，以及非平庸旋转拓扑不变量$χ^{(3)}$ = $\u202f${-2,4}$\u202f$，对应量子化分数角电荷$4e/3$。自旋轨道耦合打破了维持自旋简并的对称性$[C_{2}||M_z]$，同时保留了保护高阶拓扑相的晶体$\u27e8$C$_{3z}$$\u27e9$旋转对称性，使自旋劈裂与非平庸拓扑得以共存。此外，该高阶拓扑相在宽范围双轴应变下均保持稳定，不会出现带隙闭合和拓扑相变，证明了这种受对称性保护的拓扑态对外界微扰具有鲁棒性。本工作建立了IV型磁性系统与高阶拓扑之间的直接联系，为对称性工程化的磁性拓扑量子态研究提供了新途径。

英文摘要

Type-IV two-dimensional (2D) magnetism-a newly classified collinear magnetic phase featuring nonrelativistic spin degeneracy and spin-orbit-coupling-induced momentum-dependent spin splitting-extends the symmetry classification of collinear magnets, opening new opportunities for unconventional topological quantum states. Here, we reveal that the recently proposed two-dimensional type-IV 2D magnet MgCr$_2$O$_4$ hosts an intrinsic higher-order topological insulating phase, featuring $\mathcal{C}_{3z}$-protected corner states and a nontrivial rotational topological invariant of $χ^{(3)}$ = $\{-2,4\}$ with a quantized fractional corner charge of $4e/3$. Spin-orbit coupling breaks the spin-degeneracy-enforcing symmetry $[C_{2}||M_z]$ while preserving the crystalline $\mathcal{C}_{3z}$ rotational symmetry that protects the higher-order topological phase, thereby enabling spin splitting to coexist with the nontrivial topology. Furthermore, the higher-order topological phase remains intact throughout a wide range of biaxial strains without band-gap closing and topological phase transition, demonstrating the robustness of the symmetry-protected topological state against external perturbations. Our work establishes a direct connection between type-IV magnetic system and higher-order topology, providing a new route for symmetry-engineered magnetic topological quantum states.

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