Study of magnetic Weyl semimetals in Heusler and half Heusler alloys

非技术摘要：现代电子学的进步依赖于晶体管尺寸的缩小。纳米技术的最新进展已将硅晶体管的尺寸缩小到10纳米量级。对于这样小的晶体管来说，其他的物理效应开始起作用，限制了晶体管的功能。需要新颖的材料和设备概念。最近，一组被称为Heusler合金的材料在理论上被预测有应用前景。这些材料是铁磁性的，有望承载一种最近在固体材料中发现的新型无质量基本粒子，称为Weyl费米子。这一努力将为赫斯勒合金中这种奇异状态提供实验证据。如果成功，它不仅将推进拓扑Weyl半金属的知识，而且还可能导致自旋电子学的应用。该项目还为主要研究人员提供了一个机会，加强在晶体材料发现和生长领域的教育和培训，这在国家研究委员会的报告“晶体物质前沿”中得到了强调。技术摘要：近年来在非磁性材料（如taas类材料、光子晶体和（W/Mo）Te2）中发现的拓扑Weyl半金属引起了极大的兴趣并引起了全世界的关注。然而，铁磁Weyl半金属尚未在实验中实现，尽管它们被预测具有更令人兴奋的性质，例如单层铁磁Weyl半金属中的量子反常霍尔效应。在预测铁磁性Weyl半金属方面已经进行了大量的理论研究，并提出了许多候选材料。本研究的主要目的是通过实验验证最近预测的候选材料体系，即Heusler合金Co2XZ（X=V, Zr, Nb, Ti, Mn, Hf; Z =Si, Ge, Sn， Ga和Al）。本项目主要研究人员利用浮区、金属通量和化学气相输运等方法生长Co2XZ族各种成员的单晶，并通过磁输运、霍尔效应、量子振荡和光电光谱实验寻找这些材料中Weyl费米子的输运和光谱证据。此外，主要研究人员还研究了几种半Heusler合金体系ReMX （Re=稀土，MX=PdBi和pdbb）在磁场驱动下的Weyl态。本部分研究的目标是寻找更多的场驱动Weyl半金属态，以此为平台验证所提出的场驱动Weyl态的机制，并研究Weyl态对自旋轨道耦合的依赖关系。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Non-technical abstract:The advancement of modern electronics has relied on the shrinking of transistor size. Recent progress in nanotechnology has reduced the size of silicon transistors down to the order of 10 nanometers. For such small transistors, other physical effects set in, limiting the functionality of the transistors. Novel materials and device concepts are required. Recently, a group of materials called Heusler alloys has been theoretically predicted to be promising for applications. These materials are ferromagnetic and expected to host a new type of massless fundamental particles discovered recently in solid materials, called Weyl fermions. This effort will provide experimental evidence for such exotic states in Heusler alloys. If successful, it would not only advance the knowledge of topological Weyl semimetals, but also likely lead to applications in spintronics. This project also provides the principal investigators with an opportunity to strengthen the education and training in field of discovery and growth of crystalline materials, the importance of which was highlighted in the National Research Council's report "Frontiers in Crystalline Matter". Technical abstract:Recent discoveries of topological Weyl semimetals in non-magnetic materials such as TaAs-class materials, photonic crystals and (W/Mo)Te2 have generated immense interest and attracted worldwide attention. However, ferromagnetic Weyl semimetals have not been experimentally realized yet, though they were predicted to have more exciting properties, e.g. quantum anomalous Hall effect in monolayer ferromagnetic Weyl semimetal. There have been considerable theoretical efforts in predicting ferromagnetic Weyl semimetals and many candidate materials have been proposed. The major objective of this proposed research is to experimentally verify one recently-predicted candidate material system, i.e. Heusler alloy Co2XZ(X=V, Zr, Nb, Ti, Mn, Hf; Z =Si, Ge, Sn, Ga and Al). The principal investigators of this project grow single crystals of various members of the Co2XZ family using the floating-zone, metal flux and chemical vapor transport methods and seek transport and spectroscopic evidences of Weyl fermions in these materials through magneto-transport, Hall effect, quantum oscillation and photoemission spectroscopy experiments. In addition, the principal investigators also study the Weyl states driven by a magnetic field in several half Heusler alloy systems ReMX (Re= rare earth, MX=PdBi and PtSb). The goal of this part of research is to find more field-driven Weyl semimetal states, which could be used as a platform to verify the proposed mechanisms for the field-driven Weyl state and study the dependence of Weyl state on spin-orbital coupling.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.