Orbital Magnetism and Related Phenomena in Topological Metals

在过去的十年里，拓扑材料得到了广泛的研究，因为它们的电子态性质与传统材料有很大的不同，从而产生了新的效应。能带拓扑可以用拓扑不变量来描述，其整数值在光滑变形下保持不变，直到系统经历与体能带结构不连续变化相关的拓扑相变。此外，拓扑材料拥有能够传导无耗散自旋极化电流的表面态，这使得它们在不久的将来有可能成为自旋电子学应用的候选者。在大多数现有的自旋电子学应用中，通常是磁化的自旋部分受到控制和操纵。近年来，人们认识到利用电学手段可以在某些非磁性材料中诱导出大的轨道矩，并且这种效应与能带拓扑结构有关。在拓扑绝缘体中，电场可以诱导纯轨道磁化，与传统的磁电材料相比，线性磁电(ME)系数量化到非常大的值。电流的通过也可以在某些类别的非磁性金属中诱导出大的轨道矩。这种动力学ME效应的一个先决条件是晶体结构应该没有反转中心。破缺的逆对称性也是一类重要的拓扑Weyl半金属的特征，使它们成为实现这种ME效应的很好的候选者。在上述两个例子中，在基态不具有磁序的系统中，磁感应是由电方法引起的，这为新出现的自旋轨道电子学领域开辟了很有希望的可能性。该计划的目的是研究拓扑材料中的轨道磁性现象，特别是外部微扰可以诱导轨道矩的方式，以及探测非平凡带拓扑的相关光学效应。在前人对拓扑绝缘体、Weyl半金属和节线半金属的电学性质研究的基础上，我们将估算这些体系的磁化轨道部分和光学响应函数。将特别强调半金属相，这是最近出现的一个主要研究主题，其响应特性目前正受到密切关注。这项工作的结果将使人们能够识别出可以在自旋轨道电子学设备中充当可切换组件的现实材料。

Topological materials have been intensively studied over the past decade, because the properties of their electronic states differ in fundamental ways from those of conventional materials, giving rise to new effects. The band topology can be characterized by topological invariants, whose integer values remain unchanged under smooth deformations, until the system undergoes a topological phase transition associated with discontinuous changes in the bulk band structure. Additionally, topological materials host surface states capable of conducting dissipationless spin polarized currents, making them promising candidates for near-future spintronics applications.In most existing spintronics applications, it is usually the spin part of the magnetization that iscontrolled and manipulated. In recent years, it was realized that large orbital moments can be induced in certain classes of nonmagnetic materials by electrical means, and that such effects can be related to the band topology. In topological insulators, an electric field can induce a purely orbital magnetization, with the linear magnetoelectric (ME) coefficient quantized to a very large value compared to conventional magnetoelectrics. Large orbital moments can also be induced in certain classes of nonmagnetic metals by the passage of an electrical current. A prerequisite for this kinetic ME effect is that the crystal structure should lack a center of inversion. Broken inversion symmetry is also a characteristic feature of the important class of topological Weyl semimetals, making them good candidates for realizing this ME effect. In both above examples, magnetism is induced by electrical means in systems that do not possess magnetic order in their ground state, opening up promising possibilities in the emerging field of spin-orbitronics.The aim of the proposed project is to investigate the phenomenon of orbital magnetism in topological materials, particularly the ways in which orbital moments can be induced by external perturbations, as well as related optical effects that probe the nontrivial band topology. Building on previous successful investigations of the electronic properties of topological insulators, Weyl semimetals and nodal-line semimetals, the orbital part of the magnetization and the optical response functions will be evaluated for these systems. Special emphasis will be placed on the semimetallic phases, which have recently emerged as a major research topic, with their response properties in particular being currently under intense scrutiny. The results of this work will allow to identify realistic materials that could act as switchable components in spin-orbitronics devices.