[论文解读] Tuning Fermi Levels in Intrinsic Antiferromagnetic Topological Insulators MnBi2Te4 and MnBi4Te7 by Defect Engineering and Chemical Doping

Mao‐Hua Du, Jiaqiang Yan+2 more|arXiv (Cornell University)|Jun 27, 2020

Topological Materials and Phenomena被引用 6

一句话总结

本研究证明，缺陷工程与化学掺杂可有效调节本征反铁磁拓扑绝缘体MnBi2Te4和MnBi4Te7中的费米能级。通过控制生长条件（Te富集与Te贫乏）并引入NaMn受主，费米能级被移至禁带内，抑制了金属导电性，并实现了拓扑表面态的观测。

ABSTRACT

MnBi2Te4 and MnBi4Te7 are intrinsic antiferromagnetic topological insulators, offering a promising materials platform for realizing exotic topological quantum states. However, high densities of intrinsic defects in these materials not only cause bulk metallic conductivity, preventing the measurement of quantum transport in surface states, but may also affect magnetism and topological properties. In this paper, we show by density functional theory calculations that the strain induced by the internal heterostructure promotes the formation of large-size-mismatched antisite defect BiMn in MnBi2Te4; such strain is further enhanced in MnBi4Te7, giving rise to even higher BiMn density. The abundance of intrinsic BiMn donors results in degenerate n-type conductivity under the Te-poor growth condition. Our calculations suggest that growths in a Te-rich condition can lower the Fermi level, which is supported by our transport measurements. We further show that the internal strain can also enable efficient doping by large-size-mismatched substitutional NaMn acceptors, which can compensate BiMn donors and lower the Fermi level. Na doping may pin the Fermi level inside the bulk band gap even at the Te-poor limit in MnBi2Te4. Furthermore, facile defect formation in MnSb2Te4 and its implication in Sb doping in MnBi2Te4 as well as the defect segregation in MnBi4Te7 are discussed. The defect engineering and doping strategies proposed in this paper will stimulate further studies for improving synthesis and for manipulating magnetic and topological properties in MnBi2Te4, MnBi4Te7, and related compounds.

研究动机与目标

• 解决MnBi2Te4和MnBi4Te7中高本征缺陷密度导致的体相金属导电性问题。
• 克服费米能级在导带附近钉扎导致的拓扑表面态测量受抑制问题。
• 开发将费米能级移至体相禁带内的策略，以实现量子输运测量。
• 研究内应力与缺陷形成在实现大尺寸失配离子有效掺杂中的作用。
• 探索通过MnSb2Te4和MnBi4Te7中缺陷偏析实现Sb掺杂的可行性，以实现进一步调控。

提出的方法

• 采用密度泛函理论（DFT）计算分析MnBi2Te4和MnBi4Te7中的缺陷形成能与电子结构。
• 模拟内异质结构引起的应变对MnBi2Te4和MnBi4Te7中反位缺陷（BiMn）形成的影响。
• 模拟Te富集与Te贫乏生长条件对费米能级位置与载流子浓度的影响。
• 评估NaMn取代掺杂作为受主以补偿BiMn施主并降低费米能级的可行性。
• 通过分析MnSb2Te4及其相关固溶体，研究缺陷偏析与Sb掺杂路径。
• 将理论预测与实验输运测量结果相关联，以验证费米能级的调控。

实验结果

研究问题

• RQ1 异质结构引起的内应力在多大程度上影响MnBi2Te4和MnBi4Te7中BiMn反位缺陷的形成能？
• RQ2 Te富集生长条件是否能有效降低MnBi2Te4和MnBi4Te7中的费米能级，以实现本征半导体行为？
• RQ3 NaMn受主在多大程度上能补偿BiMn施主并使费米能级移至体相禁带内？
• RQ4 缺陷偏析在MnBi4Te7中的作用是什么？其如何影响电子与磁性性质？
• RQ5 通过MnSb2Te4实现Sb掺杂是否可作为调节MnBi2Te4中费米能级的可行途径？

主要发现

• MnBi2Te4和MnBi4Te7中的内应力显著降低了BiMn反位缺陷的形成能，导致高本征施主浓度。
• Te贫乏生长条件导致因大量BiMn施主的存在而出现简并n型导电性，费米能级被钉扎在导带附近。
• Te富集生长条件被预测可降低费米能级，实验输运测量结果证实了该预测。
• NaMn掺杂作为有效受主，可补偿BiMn施主，并在Te贫乏条件下仍将费米能级移至体相禁带内。
• 理论计算表明，Na掺杂可将MnBi2Te4的费米能级钉扎在带隙内，从而实现拓扑表面态的观测。
• 识别出缺陷偏析与Sb掺杂路径为调控这些材料中电子与磁性性质的可行策略。