三维块体外尔半金属的反常能斯特效应研究

Anomalous Nernst effect (ANE) is that a transverse voltage can be generated by a longitudinal temperature gradient even in absent of magnetic field. This effect has been observed and investigated in conventional ferromagnets and its signal was believed to scale with spontaneous magnetization. However, topological Weyl semimetals associated with the chiral magnetic effect does not follow the scale relation. Thus the study of the mechanism of ANE as well as the relationship between ANE and Berry curvature is highly desirable. In this project, we will focus on the magnetic Co3Sn2S2 and non-magnetic NbP(As) topological Weyl semimetals, and through the measurement of Nernst effect, we will investigate: (1) By measuring the quantum oscillations of Nernst effect under high magnetic fields, giving the information of the Fermi Surface topological structures. (2) Measuring the Nernst effect in both systems, discovering the origin of giant anomalous Nernst effect and the relationship between ANE and magnetization, and then exploring the intrinsic topological Weyl state as well as the relationship of Nernst effect and non-trivial Berry curvature; (3) By investigating the anomalous Nernst effect, quantificationally describing the Nernst signal originated from the contributions of ferromagnetic order and Berry curvature, respectively, and revealing the difference and the relationship between them.

反常能斯特效应是指在外磁场为零时，施加纵向的温度梯度也能够产生一个横向的电压。它已经在传统铁磁体中被观察和研究，并被认为反常能斯特信号正比于自发磁化强度。然而，具有手性磁效应的拓扑外尔半金属体系可能不遵循这个关系，其反常能斯特效应的机理以及与Berry曲线的联系都亟待探索研究。本项目以磁性Co3Sn2S2和非磁性NbP(As)两类外尔半金属为研究对象，通过系统的能斯特效应测量：(1)研究样品强磁场下的Nernst效应量子震荡行为，获取费米面拓扑结构信息；(2) 研究两类外尔半金属的能斯特效应，揭示巨大反常能斯特效应的来源及其与磁化强度的函数关系，探明体系内秉的拓扑Weyl态以及与Berry曲线的内在联系；(3) 定量分析铁磁态和非平庸Berry曲线对反常能斯特信号的贡献，揭示拓扑Weyl体系与传统铁磁体产生反常能斯特效应的区别和联系。

拓扑量子材料展现出奇异的量子现象而成为凝聚态物理研究的热点，尤其是磁性拓扑材料，探索磁性与拓扑的内在关联是揭示许多反常量子现象的关键。本项目主要聚焦磁性拓扑半金属的反常热电效应研究，探索体系中反常热电效应与磁有序、拓扑电子态之间的内在关联和影响。我们重点研究了磁性拓扑半金属体系Co3Sn2S2、不同Fe空位的Fe3-xGeTe2体系，以及非磁性拓扑半金属W2As3体系的反常输运性质，取得了较大的研究成果。主要成果如下：.1. 率先报道了磁性Weyl半金属Co3Sn2S2的巨大反常能斯特效应，发现体系的Nernst信号严重偏离常规铁磁体能斯特信号正比于磁化强度的标度关系， 比传统铁磁体的能斯特信号大1-2个数量级。研究成果有助于理解拓扑与磁性之间的内在联系。.2. 研究了不同Fe空位Fe3-xGeTe2样品的反常能斯特效应随磁场和温度的依赖关系，发现Fe空位可以调控反常霍尔效应机制，经历从Hopping区域到内禀机制过渡，同时反常能斯特信号的符号发生了改变。结合第一性原理计算表明该现象与手性Weyl点的类型、化学势平移和非平庸Berry曲率存在密切关系。.3. 系统测量了W2As3的角分辨磁阻，发现其在30K附近发生了突变，结合能斯特效应、热电势和Hall效应数据，证实了拓扑半金属W2As3在30K附近经历了温度诱导的Lifshitz转变。我们首次利用角分辨磁阻来探索拓扑相变，是一种简单易行且普适的方法。.4. 研究了 Kitaev自旋液体候选材料Li3Co2SbO6正交相多晶粉末样品进行了直流和交流磁化率、缪子自旋弛豫(mSR)和中子衍射等综合表征，揭示了正交相Li3Co2SbO6的奇异磁性质，填补了对正交相磁基态研究的空缺。.在本项目资助下共发表高水平SCI论文8篇：中科院一区论文3篇，二区论文5篇；培养硕士研究生3名，参加学术会议4次，邀请报告1次。