Study of magnetic Weyl semimetals in Heusler and half Heusler alloys

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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.