磁性拓扑材料的电子结构研究

磁性拓扑材料的研究是当前凝聚态物理领域中备受关注的重要课题之一。 磁性拓扑材料可以分为两大类：磁性掺杂拓扑材料和本征磁性拓扑材料。它们 具有引人注目的量子性质和广阔的应用前景。这些新奇的量子性质往往与电子 结构息息相关，因此深入研究磁性拓扑材料的电子结构将有助于揭示新奇的物 理现象，探索新颖的量子态，深化对物理学的理解，并为未来量子计算和自旋电 子学领域上的应用开辟新的可能性。本文首先介绍了拓扑概念在凝聚态物理领 域的发展历程；其次回顾了磁性拓扑材料的研究现状；然后根据研究现状总结了 现存问题：本征磁性拓扑材料的能带表征问题、磁性拓扑材料的设计与合成问题 和磁性掺杂对拓扑材料的性质调控问题；最后基于以上问题展开研究。在各项研 究中均采用了角分辨光电子能谱技术（Angle-resolved Photoemission Spectroscopy， ARPES，一种基于光电效应探究固体电子结构的技术）对材料的电子结构进行 表征。下面介绍本论文的主要研究。 （1）本征磁性拓扑绝缘体 MnBi2Te4 的表面态研究：MnBi₂Te₄ 作为首个本征 反铁磁拓扑绝缘体，其电子结构的测量结果具有争议。早期的文献报道，通过 ARPES 观测到 MnBi₂Te₄ 表面态存在与理论计算相当的磁能隙。但是，该能隙 在高温顺磁相中依然存在，这与理论预期不符。为深入探究这一问题，本文使 用高动量-高能量分辨的激光 ARPES 测量其电子结构，结果显示 Γ 点处存在无 能隙的 Dirac 型能带。通过温度依赖实验，确定了该 Dirac 型能带在反铁磁相和 顺磁相中均未打开能隙。通过变光子能量的研究，确定了这种无能隙 Dirac 型 能带是表面态，证明了此前报道的有能隙表面态实为 MnBi₂Te₄ 的体态能带。通 过表面扰动实验，说明了 MnBi₂Te₄ 的表面态类似非磁性拓扑绝缘体的表面态， 都具有鲁棒性。此外，通过自旋分辨 ARPES 实验，揭示了 MnBi₂Te₄ 的表面态呈 现“刺猬型”自旋纹理。最后，通过结合理论计算与实验结果，提出了 MnBi₂Te₄ 的无能隙 Dirac 型表面态是由样品表面的磁无序引起的可能性。首次在反铁磁 拓扑材料中观测到无能隙表面态，这一发现改变了人们对拓扑量子态和对称保 护机制的传统认知，揭示了反铁磁拓扑材料中可能存在的新物理机制。 （2）本征磁性拓扑半金属候选材料 V₃S₄ 的合成与其电子结构受自旋轨道 耦合作用影响大小的研究：自旋轨道耦合作用是理论计算电子结构时的重要参 数。它能够在保持时间反演对称性下引起能带反转，从而导致非平庸的拓扑性质。然而，自旋轨道耦合作用通常在含有高原子序数元素（如 Bi、Pb、Hg 等） 的材料中才显著，这限制了拓扑材料的设计。理论计算表明，V₃S₄（其单晶未 曾被合成过）是一种磁性拓扑半金属，并且自旋轨道耦合对其电子结构具有显 著影响。第一性原理计算表明，在不考虑自旋轨道耦合的情况下，在 V₃S₄ 中同 时存在第二类 Dirac 半金属态和拓扑节线半金属态；考虑自旋轨道耦合作用后， 第二类 Dirac 点处打开能隙，拓扑节线消失。为了验证V₃S₄ 的电子结构是否与 理论计算一致，本文通过 ARPES 实验，确定了 V3S4 在布里渊区中 Γ-Y 高对称 线上的第二类 Dirac 锥打开了约 120 meV 的能隙；在 I-Z-I 高对称线上呈现节线 消失后的能带结构。此实验，证明了 V3S4 的实际能带结构与考虑自旋轨道耦 合作用计算的能带结构一致。最后，通过结合理论计算与实验结果，揭示了自 旋轨道耦合作用对由低原子序数元素构成的 V₃S₄ 材料的能带结构有显著影响， 这一发现将有助于发现更多由低原子序数元素组成的拓扑材料。 （3）磁性掺杂调控拓扑半金属 La_1−xCe_xBi 中极大磁阻效应的研究：极大磁 阻（XMR）现象已在许多拓扑材料中被观测到，但其机制尚无定论。为探究 XMR 效应的本质机理，首先通过助熔剂法生长了不同掺杂浓度的 La_1−xCe_xBi单晶。 随后，通过测量其输运性质，发现了随着 Ce 掺杂量的增加，La_1−xCe_xBi 体系中 的 XMR 现象逐渐被抑制；在 12 T 磁场和 2 K 温度下，La_1−xCe_xBi（x >0.09）的 磁阻值小于 10³%（通常 XMR 效应的磁阻大于 10³%）。通过拟合纵向电导率和 霍尔电导率，确定了在 XMR 效应被抑制的 La_0.88Ce_0.12Bi 中，电子-空穴载流子 浓度依然补偿且迁移率依然较高，这表明了电子-空穴补偿和高迁移率并非实现 XMR 效应的充分条件。通过对比未掺杂的 LaBi 和高掺杂的 La_0.7Ce_0.3Bi 单晶的 ARPES 实验结果，确定了掺杂前后费米面的位置没有移动，表面态没有打开能 隙，且 d-p 轨道杂化等特征能带结构依然存在，这表明 XMR 效应与费米面附 近的电子结构无直接关联。此项研究为理解 XMR 效应的起源提供了重要约束 条件，并为进一步探索 XMR 效应的起源提供了新的研究方向。

Research on magnetic topological materials is currently one of the most prominent topics in condensed matter physics. These materials can be divided into two categories: magnetic-doped topological materials and intrinsic magnetic topological materials. They exhibit intriguing quantum properties and have broad application potential. These novel quantum properties are often closely related to electronic structures; therefore, in-depth studies of the electronic structures of magnetic topological materials are essential for uncovering new physical phenomena, exploring novel quantum states, deepening our understanding of physics, and opening up new possibilities for future applications in quantum computing and spintronics. This thesis begins with an introduction to the development of topological concept in condensed matter physics. It then reviews the current research status of magnetic topological materials and summarizes existing issues, including the challenges of band characterization in intrinsic magnetic topological materials, the difffculties in designing and synthesizing magnetic topological materials, and the control of topological properties through magnetic doping. The study is conducted with a focus on these issues. Throughout the research, Angle-resolved Photoemission Spectroscopy (ARPES), a technique based on the photoelectric effect used to probe the electronic structure of solids, is employed to measure the electronic structures of the materials. The main research objectives of this thesis are outlined below. (1) Study on the Surface States of the Intrinsic Magnetic Topological Insulator MnBi₂Te₄：MnBi₂Te₄ is the ffrst known intrinsic antiferromagnetic topological insulator, yet its electronic structure has not been systematically characterized, and there remains controversy over the measurement results of its surface states. Early literature suggested that an energy gap consistent with the theory was observed in the surface states of MnBi₂Te₄ using ARPES. However, this gap persists in the high-temperature paramagnetic phase, contradicting theoretical expectations. To investigate this further, high momentum and high energy resolution laser ARPES were employed to measure its electronic structure. The results revealed a gapless Dirac-like band at the Γ point. Temperature-dependent experiments conffrmed that this Dirac-like band does not open a gap in either the antiferromagnetic or paramagnetic phases. Studies with various photon energies conffrmed that this gapless Dirac-like band is indeed a surface state, demonstrating that the previously reported gapped surface state was actually a bulk band of MnBi₂Te₄. Surface perturbation experiments showed that the surface states of MnBi₂Te₄ exhibit topological protection robustness similar to non-magnetic topological insulators. Moreover, spin-resolved ARPES experiments revealed a ”hedgehog-like” spin texture for these surface states. Combining theoretical calculations with experimental results, it was proposed that the gapless Dirac-like surface state of MnBi₂Te₄ is due to magnetic disorder on the sample surface. This observation of a gapless surface state in an antiferromagnetic topological material refreshes conventional understandings of topological quantum states and symmetry protection mechanisms, revealing potential new physical mechanisms in antiferromagnetic topological materials. (2) Synthesis and Spin-Orbit Coupling Inffuence on the Electronic Structure of the Intrinsic Magnetic Topological Semimetal Candidate V₃S₄：Spin-orbit coupling effect is a crucial factor in topological materials. As it can induce band inversion under time-reversal symmetry, leading to non-trivial topological properties. However, spin-orbit coupling effect is typically signiffcant only in materials composed of high-Z elements (such as Bi, Pb, Hg, etc.), limiting the design of topological materials. Theoretical calculationssuggest that V₃S₄ (which has not yet been synthesized as a single crystal) is a magnetic topological semimetal, and spin-orbit coupling effect has a substantial impact on its electronic structure. First-principles calculations indicate that without spin-orbit coupling effect, both type-II Dirac semimetal state and topological nodal-line semimetal state exists in V₃S₄. When spin-orbit coupling effect is considered, the type-II Dirac point opens a gap, and the topological nodal line disappears. To verify whether the electronic structure of V₃S₄ is consistent with theoretical calculations, ARPES experiments had been conducted on this material. The result shows that the type-II Dirac cone in V₃S₄ opens a gap of about 120 meV along the Γ-Y high-symmetry line in the Brillouin zone, and no nodal lines are observed along the I-Z-I high-symmetry line. This conffrms that the actual band structure of V₃S₄ aligns with the theoretical predictions when SOC is considered. Combining theoretical calculations with experimental results, it is revealed that spin-orbit coupling effect signiffcantly affects the band structure of V₃S₄, a even though it is composed of low-Z elements. This ffnding could aid in discovering more topological materials made from low-Z elements. (3) Study on the Extremely Large Magnetoresistance Effect in the Topological Semimetal La_1−xCe_xBi Tuned by Magnetic Doping：The extremely large magnetoresistance (XMR) phenomenon has been observed in many topological materials, but its mechanism remains unresolved. To explore the intrinsic mechanism of the XMR effect, single crystals of La_1−xCe_xBi with varying doping concentrations were grown using the ffux method. Systematic measurements of their transport properties showed that with increasing Ce doping, the XMR phenomenon in the La_1−xCe_xBi system is gradually suppressed. At a magnetic ffeld of 12 T and a temperature of 2 K, the magnetoresistance value of La_1−xCe_xBi (x > 0.09) is less than 10³% (where typically, magnetoresistance values for XMR effects are greater than 10³%). Fitting the transverse resistivity and Hall resistivity determined that in La_0.88Ce_0.12Bi, where XMR is suppressed, electron-hole compensation still exists, and mobility remains high, indicating that electron-hole compensation and high mobility are not sufffcient conditions for XMR. A comparison of ARPES results from undoped LaBi and highly doped La_0.7Ce_0.3Bi single crystals conffrmed that the Fermi level does not shift, no gap opens in the surface states, and characteristic band structures such as d-p  orbital hybridization remain, suggesting that the XMR phenomenon is not directly related to the electronic structure near the Fermi level. This study provides important constraints for understanding the origin of the XMR effect and offers new directions for further exploration of its physical origins.

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