磁性拓扑绝缘体 MnBi2Te4(Bi2Te3)n的磁性和电输运性质研究

MAGNETIC AND TRANSPORT PROPERTIES IN MAGNETIC TOPOLOGICAL INSULATORS MnBi2Te4(Bi2Te3)n

孙艺琛

11849407

硕士

物理学

赵悦

2020-05-28

2020-05-28

哈尔滨工业大学

深圳

磁学和拓扑学的结合是凝聚态物理学的一个关键研究课题，它为探索一系列新兴物理学现象提供了巨大的机会，例如量子反常霍尔效应（quantum anomalous hall effect，QAHE）、轴子电动力学和马约拉纳费米子等。然而，由于缺乏合适的研究材料，这些奇异的物理效应在实验上很少能够得到实现。现如今，范德瓦尔斯层状 MnBi2Te4 等一系列相关材料问世，它们显示出了层间铁磁和层间反铁磁交换相互作用。在 MnBi2Te4 中发现了非常丰富的拓扑量子态，具有非常突出的优异特性，包括它是一种反铁磁拓扑绝缘体，其表面具有我们长期寻找的拓扑轴子态，它是具有一对外尔（Weyl）点的 II 型磁性 Weyl 半金属，以及在偶数层和奇数层薄膜中的本征轴子绝缘体和量子反常霍尔绝缘体的集合等。如果这些特性在实验上得到证实，可能会深刻地改变拓扑量子物理的未来研究和技术。在MnBi2Te4 薄层的铁磁状态下，量子反常霍尔电导的观察引起了人们的广泛关注。然而，由于大的反铁磁层间交换耦合作用，需要通过外加强磁场来实现磁矩的完全极化。本课题系统地研究了具有范德瓦尔斯耦合的磁性拓扑绝缘体MnBi4Te7 和 MnBi6Te10 单晶的磁学和电学输运性质。将生长的单晶样品利用 X射线衍射进行初步的分析与表征，然后通过一系列实验操作对样品进行封装和电极制备，进而运用综合物性测量系统（Physical Property Measurement System，PPMS）进行单晶磁性能和电输运性质的测量。之后对实验结果进行了充分探讨，分析了观察到的现象以及其中的机理，认为在 MnBi4Te7 和 MnBi6Te10 晶体中，非磁性 Bi2Te3 五元层的插入大大减弱了层间反铁磁交换耦合，进而导致反铁磁转变温度的降低，两种晶体的转变温度 TN 分别在 12.5 K 和 11 K 左右；在低温下，沿 c 轴方向施加的 0.1 T~0.2 T 的小磁场时，这两种晶体都发生了自旋翻转跃迁；由于弱的层间反铁磁交换耦合和磁晶各向异性，两种晶体在低温下（5 K 以下）都会出现铁磁迟滞现象，这种铁磁迟滞现象表明，即使在零场下，完全极化的铁磁状态也可以保持稳定。这对于实现量子反常霍尔效应是相当关键的。此外，对于 MnBi4Te7 和 MnBi6Te10 单晶，因为其具有的范德瓦尔斯结构，可以通过机械剥离的方法来实现 MnBi2Te4 七元层（septuple layer ，SLs）和 Bi2Te3五元层（quintuple layer，QLs）的各种叠加序列，因此 MnBi2Te4(Bi2Te3)n 可以通过灵活地调整堆叠顺序为研究拓扑相提供了理想的平台。

The combination of magnetism and topology is a key research topic in condensed matter physics, providing a great opportunity to explore a range of emergent physical phenomena, such as the quantum anomalous hall effect (QAHE), axion electrodynamics, and Majorana fermions. However, due to the lack of suitable research materials, these strange physical effects are rarely realized experimentally. Nowadays, a series of van der Waals layered materials related to MnBi2Te4 have appeared, which show the interlayer ferromagnetic and interlayer anti-ferromagnetic exchange interactions. Abundant topological quantum states have been found in MnBi2Te4, which have outstanding characteristics. These include an anti-ferromagnetic topological insulator, a topological axion state on the surface that have long sought, a type II magnetic Weyl Semimetal with a pair of Weyl point, and a collection of intrinsic axion insulators and quantum anomalous hall insulators in even -and odd-layer films. If these properties are confirmed experimentally, they may profoundly change future research and techniques in topological quantum physics.In the forced ferromagnetic state of MnBi2Te4 thin layer, the observation of quantum anomalous hall conductance has attracted wide attention. However, due to the large antiferromagnetic interlayer exchange coupling, it is necessary to strengthen the external magnetic field to realize the complete polarization of magnetic moment. In this paper, the magnetic and electrical transport properties of MnBi4Te7 and MnBi6Te10 single crystals of magnetic topological insulators with van der Waals coupling are systematically studied. The single crystal sample was preliminarily analyzed and characterized by X-ray diffraction, and then the sample was packaged and prepared by a series of experimental operations, and the magnetic properties and electrical transport properties of the single crystal were measured by Physical Property Measurement System (PPMS). Then the experimental results were fully discussed, and the observed phenomena and the mechanism were analyzed. It is believed that in MnBi4Te7 and MnBi6Te10 crystals, the insertion of non-magnetic Bi2Te3 quintuple layers greatly weakens the coupling of anti-ferromagnetic exchange between layers, which leads to the decrease of anti-ferromagnetic transition temperature. The transition temperature TN of the two crystals is about 12.5 K and 11K respectively. At low temperature, when a small magnetic field of 0.1 T-0.2 T is applied along the direction of the c axis, both of these crystals have a spin-flip transition. Due to the weak interlayer antiferromagnetic exchange coupling and magnetocrystal anisotropy, both crystals exhibit ferromagnetic hysteresis at low temperature (below 5 K), which indicates that a fully polarized ferromagnetic state can remain stable even at zero field. This is critical to the realization of the quantum anomalous hall effect.In addition, for MnBi4Te7 and MnBi6Te10 single crystals, due to their van der Waals structure, various superposition sequences of MnBi2Te4 septuple layer (SLs) and Bi2Te3 quintuple layer (QLs) can be realized by mechanical exfoliation. Therefore, MnBi2Te4(Bi2Te3)n provides an ideal platform for studying topological phases by adjusting the stacking order flexibly.

磁性拓扑绝缘体 MnBi2Te4(Bi2Te3)n 输运性质 量子反常霍尔效应

magnetic topological insulator MnBi2Te4(Bi2Te3)n transport properties quantum anomalous hall effect

中文

联合培养

南方科技大学

孙艺琛. 磁性拓扑绝缘体 MnBi2Te4(Bi2Te3)n的磁性和电输运性质研究[D]. 深圳. 哈尔滨工业大学,2020.