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经将硅晶体管的尺寸减小到10纳米的量级。对于这样的小晶体管，其他物理效应开始，限制了晶体管的功能。需要新的材料和设备概念。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，预计将拥有一种新型的无质量基本粒子，最近在固体材料中发现，称为Weyl费米子。这一努力将为Heusler合金中的这种奇异态提供实验证据。如果成功的话，它不仅会推进拓扑Weyl半金属的知识，而且可能会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加强晶体材料发现和生长领域教育和培训的机会，国家研究理事会的报告“晶体物质前沿”强调了这一点的重要性。最近在TaAs类材料、光子晶体和（W/Mo）Te 2等非磁性材料中发现的拓扑Weyl半金属引起了人们极大的兴趣并引起了全世界的关注。然而，铁磁Weyl半金属尚未在实验上实现，尽管它们被预测具有更多的激发性质，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Weyl半金属方面已经做了大量的理论工作，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系统，即Heusler合金Co2 XZ（X=V，Zr，Nb，Ti，Mn，Hf; Z =Si，Ge，Sn，Ga和Al）。该项目的主要研究人员使用浮区法、金属助熔剂法和化学气相输运法生长Co2 XZ家族的各种成员的单晶，并通过磁输运、霍尔效应、量子振荡和光电子能谱实验来寻找这些材料中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个半Heusler合金系统ReMX（Re=稀土，MX=PdBi和PtSb）在磁场驱动下的外尔态。这部分研究的目标是找到更多的场驱动外尔半金属态，并以此作为验证场驱动外尔态的机制和研究外尔态对自旋轨道耦合的依赖性的平台。该奖项反映了NSF的法定使命，并通过使用基金会的智力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.