磁性拓扑绝缘体Mn(Bi1-xSbx)2Te4的自旋分辨角分辨光电子能谱研究

本征反铁磁拓扑绝缘体 MnBi₂Te₄ 因有望实现量子反常霍尔效应和轴子绝缘体相引发了学界对该体系的广泛研究。不同于理论预言和早期研究中报道的有能隙的拓扑表面态，高分辨率的基于激光光源的角分辨光电子能谱研究明确指出 MnBi₂Te₄ 的 (0001) 面存在无能隙的狄拉克锥。对 Sb 掺杂的 MnBi₂Te₄ 的角分辨光电子能谱研究进一步指出 Sb 掺杂不仅导致能带表现出了空穴掺杂行为，还使得表面态在狄拉克点打开了一个对体态磁性转变不敏感的能隙，且该能隙的大小随着掺杂浓度的增加而增大。在这里，我们使用助溶剂法生长了高质量的 Sb 掺杂 的 MnBi₂Te₄ 单晶样品，利用基于同步辐射光源的自旋分辨角分辨光电子能谱研究了 Sb 掺杂的 MnBi₂Te₄ 能带的自旋极化，并在其表面的中心高对称点处观测到了面外 z 方向的自旋翻转，这与我们随后的圆二色性角分辨光电子能谱实验中观察到的掺杂导致的二色性信号在能隙上下反号的现象一致。我们的研究结果指出 Mn(Bi_1-xSb_x)₂Te₄ 表面存在由 Sb 掺杂导致的磁能隙，不仅表明磁性拓扑绝缘体 Sb 掺杂的 MnBi₂Te₄ 是实现丰富拓扑相的潜在平台，也有助于进一步理解磁性拓扑体系中的磁与拓扑相互作用。

The intrinsic antiferromagnetic topological insulator MnBi₂Te₄ has triggered extensive research due to the prospect of realizing the quantum anomalous Hall effect and the axion insulator phase. In contrast to the gapped topological surface state reported in theoretical predictions and previous studies, high-resolution laser-based angle-resolved photoemission spectroscopy studies unambiguously revealed a gapless Dirac cone on the (0001) surface of MnBi₂Te₄. Furthermore, angle-resolved photoemission spectroscopy studies on Sb-doped MnBi₂Te₄ demonstrated that Sb doping not only leads to the p-doping behavior in the band but also results in a gap at the Dirac point that is insensitive to the bulk magnetism; the gap size increases with higher doping levels. Here, we synthesize high-quality Sb-doped MnBi₂Te₄ single crystals using the flux method and investigate the spin polarization of Sb-doped MnBi₂Te₄ utilizing synchrotron-based spin- and angle-resolved photoemission spectroscopy. We observe spin flipping along the out-of-plane z direction at the central high symmetry point on the surface, consistent with the circular dichroism sign inversion across the gap observed in our circular dichroism angle-resolved photoemission spectroscopy experiments. Our work points to a magnetic gap on the surface of Mn(Bi_1-xSb_x)₂Te₄ induced by Sb doping, not only indicating that the magnetic topological insulator Sb-doped MnBi₂Te₄ is a potential platform for hosting rich topological phases but also shedding light on the understanding of interactions between magnetism and topological properties in magnetic topological systems.

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