磁性拓扑绝缘体中局域杂质态对交换耦合机制的普遍影响
Ubiquitous impact of localised impurity states on the exchange coupling mechanism in magnetic topological insulators
摘要：
自从在磁性掺杂拓扑绝缘体（MTI）Cr：（Bi，Sb）$_2$ Te $_3$和V：（Bi，Sb）$_2$ Te $_3$中发现量子反常霍尔效应以来，寻找铁磁性发生的交换耦合机制一直是一个中心问题，并提出了各种不同的方案。结合共振光电子能谱、X射线磁二向色性和多重配位场论，我们确定了（Bi，Sb）$_2$ Te $_3$中V和Cr杂质的局域电子和磁性组态。虽然强的pd杂化被发现这两种掺杂剂类型，其三维态密度显示出显着的差异。最先进的第一性原理计算显示了这些杂质状态如何介导特征短程pd交换相互作用，其强度敏感地随3d状态相对于费米能级的位置而变化。不同的主机化学计量的薄膜上的测量支持这一趋势。我们的研究结果建立了必不可少的作用，杂质状态介导的交换相互作用的磁性MTI。
英文摘要：
Since the discovery of the quantum anomalous Hall effect in the magnetically doped topological insulators (MTI) Cr:(Bi,Sb)$_2$Te$_3$ and V:(Bi,Sb)$_2$Te$_3$, the search for the exchange coupling mechanisms underlying the onset of ferromagnetism has been a central issue, and a variety of different scenarios have been put forward. By combining resonant photoemission, X-ray magnetic dichroism and multiplet ligand field theory, we determine the local electronic and magnetic configurations of V and Cr impurities in (Bi,Sb)$_2$Te$_3$. While strong pd hybridisation is found for both dopant types, their 3d densities of states show pronounced differences. State-of-the-art first-principles calculations show how these impurity states mediate characteristic short-range pd exchange interactions, whose strength sensitively varies with the position of the 3d states relative to the Fermi level. Measurements on films with varying host stoichiometry support this trend. Our results establish the essential role of impurity-state mediated exchange interactions in the magnetism of MTI.
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