Design and Characterization of Novel Superconducting Topological Semimetals

摘要：固体可分为绝缘体、半金属和金属。近年来，拓扑半金属已成为电子材料研究的一个新前沿。它们所展示的新现象不仅具有根本性的意义，而且可能具有巨大的技术应用潜力。例如，超导拓扑半金属可以潜在地承载一种新型的电子态，马约拉纳束缚态，这被认为是容错量子计算的关键成分。然而，在实验室中合成如此复杂的量子材料是具有挑战性的。本项目采用先进的原子尺度技术来制造和表征超导拓扑半金属。本课题旨在深入了解拓扑半金属的生长机制和电子特性，为量子计算中的实际器件应用提供丰富的材料基础。建议的研究还旨在涉及广泛的学生，包括初中生，高中生，本科生和研究生，并为他们提供充分的机会参与充满活力的实验室活动。最后，该项目完全致力于扩大代表性不足的群体参与先进材料科学的教育和研究。技术摘要：自从在TaAs中发现第一个Weyl半金属以来，实现拓扑半金属相成为一个重要的研究兴趣。拓扑半金属扩展了拓扑材料的领域，超出了拓扑绝缘体和拓扑晶体绝缘体等绝缘相。在拓扑半金属中发现并实现了许多非凡的性质，包括无质量的Weyl费米子、手性异常、费米弧表面态和非局部电动力学。这些发现不仅具有基础科学的重要性，而且对实际设备应用也有希望。将超导性引入到拓扑半金属中，为传统超导体中无法获得的许多新现象打开了大门。拓扑半金属中拓扑体态和表面态的配对有可能产生非s波超导性，这是实现新型准粒子（如Majorana束缚态）的关键因素。然而，到目前为止，超导拓扑半金属仍然非常罕见。本项目结合分子束外延、角分辨光发射光谱和扫描隧道显微镜来合成和探索具有超导性的新型拓扑半金属。具体而言，该项目旨在在第一线原理模拟的指导下，创建新的超导拓扑半金属，如PbTaSe2， TlTaSe2, MoC, NbC, Mg2Pb（Sn）和Mg3Bi2。采用分子束外延的方法在掺杂水平可控的条件下生长超导材料，并通过光电发射和扫描隧道显微镜/光谱对其进行了表征。该项目的目标是识别奇异的超导拓扑半金属，并为从拓扑节点费米子和超导的相互作用中研究丰富的物理奠定坚实的基础。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。

Nontechnical Abstract: Solids can be classified as insulators, semimetals, or metals. Recently, topological semimetals have emerged as a new frontier in the research field of electronic materials. Novel phenomena they exhibit are not only of fundamental interest, but may hold great potential for technological applications. For example, superconducting topological semimetals can potentially host a new type of electronic state, Majorana bound state, which is believed to be the key ingredient for fault-tolerant quantum computation. However, it is challenging to synthesize such complicated quantum materials in the laboratory. This project uses advanced atomic-scale techniques to fabricate and characterize superconducting topological semimetals. The goal of this project is to achieve a thorough understanding of the growth mechanism and electronic character of topological semimetals and to provide a rich materials base for practical device applications in quantum computation. The proposed research also aims to involve a wide range of students, including middle school, high school, undergraduate and graduate students, and offer them ample opportunities to engage in vibrant lab activities. Finally, this project is fully committed to broadening participation of under-represented groups in education and research of advanced materials science. Technical Abstract: Since the discovery of the first Weyl semimetal in TaAs, a significant research interest emerged in realizing topological semimetallic phases. The topological semimetals extend the territory of topological materials beyond the insulating phases such as topological insulators and topological crystalline insulators. Many extraordinary properties have been found and realized in topological semimetals, including massless Weyl fermions, chiral anomalies, Fermi arc surface states, and nonlocal electrodynamics. Those findings are not only of fundamental scientific importance, but also hold promise for practical device applications. Introducing superconductivity to topological semimetals opens the door to many novel phenomena that are unavailable in conventional superconductors. The pairing of the topological bulk and surface states in topological semimetals can potentially generate non-s-wave superconductivity, which is the key ingredient for realizing new types of quasiparticles such as Majorana bound states. However, up to date, superconducting topological semimetals are still very rare. This project uses a combination of molecular beam epitaxy, angle-resolved photoemission spectroscopy, and scanning tunneling microscopy to synthesize and explore novel topological semimetals that intrinsically host superconductivity. Specifically, the project aims to create new superconducting topological semimetals such as PbTaSe2, TlTaSe2, MoC, NbC, Mg2Pb(Sn), and Mg3Bi2 under the guidance of first-principles simulations. The superconducting materials are grown by the method of molecular beam epitaxy at controlled doping levels and characterized through photoemission and scanning tunneling microscopy/spectroscopy. The goal of this project is to identify exotic superconducting topological semimetals and to form a solid foundation for studying rich physics from the interplay of topological nodal fermions and superconductivityThis 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.