CAREER: Study of Electronic and Magnetic Topological Phenomena in Two Dimensional Quantum Materials with Scanning Probe Microscopy

非技术描述：拓扑的数学概念，研究形状的整体几何特性，如孔或表面的数量，当物体平滑变形时保持不变，已被应用于理解和发现固态材料中的新型电子和磁性能。这个项目在只有几个原子层厚的二维材料中研究了两种类型的这种现象。第一种是具有拓扑边缘传导的二维材料。在这些材料中，样品的内部是绝缘的，而电流只能沿着边缘流动。这种边缘传导可以大大减少电流流动过程中的能量损失，因此，它可以潜在地用于开发下一代节能电子设备。第二种现象发生在二维磁性材料中，其中局部磁极以旋转的方式缠绕。绕组方向可以用于信息存储，因为它们的拓扑特性使它们对外部扰动具有鲁棒性，从而能够长时间保留信息。该项目进一步整合了一个教育组成部分，让本科生和高中生参与量子材料和纳米技术领域的多学科研究。PI将开发适合各个层次学生的研究模块，例如构建涉及硬件设计和电子开发的演示纳米表征工具。示范项目为学生参与拟议的研究活动提供了一个桥梁。教育项目将特别鼓励和招募代表性不足的群体。技术描述：原子层状材料为研究二维拓扑物理提供了丰富的平台。本项目研究了几种新型二维材料及其异质结构，以表征其拓扑状态，并探索操纵和设计其拓扑特性的新方法。第一个材料系统是最近发现的拓扑磁体MnBi2Te4，它在一种材料中结合了磁性和拓扑秩序，并且在少数层样品中是陈氏绝缘体。提出的研究旨在表征对拓扑顺序很重要的几层样品的关键性质，包括拓扑间隙和磁各向异性。第二种材料体系是基于石墨烯的波纹异质结构，包括在hBN上排列的扭曲双层石墨烯和石墨烯与过渡金属二硫族化合物的波纹异质结构界面。具有大周期性的莫尔条纹超晶格的形成有望调节石墨烯中的朗道能级结构，这可能被用于调节石墨烯的拓扑状态。第三种材料体系是二维磁体的异质结构。本研究探索了不同的材料组合，在界面处引入强自旋轨道耦合和破坏反转对称性，从而诱导Dzyaloshinskii-Moriya相互作用并在磁性材料中产生skyrmions。本项目采用微波阻抗显微镜（MIM）、静电力显微镜（EFM）和磁力显微镜（MFM）等多种扫描探针显微镜技术，探测局部电子和磁性能，并在真实空间中对拓扑状态进行成像。局部探针为传统的传输技术提供了补充信息，可以从不同的角度对二维拓扑物理产生新的见解。该研究结果将为理论学家完善模型参数和材料科学家优化和提高样品质量提供重要的反馈。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 description:The mathematical concept of topology, which studies global geometrical properties of shapes, such as the number of holes or surfaces, that remain unchanged when the objects are smoothly deformed, has been applied to understand and discover novel electronic and magnetic properties in solid state materials. This project studies two types of such phenomena in two-dimensional materials that are only a few atomic layers thick. The first is 2D materials that host topological edge conduction. In these materials, the interior of sample is insulating while electrical current can only flow along the edges. This kind of edge conduction can greatly reduce the energy loss during the current flow, therefore, it can potentially be used to develop next generation of energy efficient electronic devices. The second type of phenomena occurs in 2D magnetic materials where the local magnetic poles wind around in a swirling pattern. The winding directions can be used for information storage as their topological properties make them robust against external perturbation thus capable of retaining information for longer time. This project further integrates an education component to engage both undergraduate and high school students in multidisciplinary research in the field of quantum materials and nanotechnology. The PI will develop research modules that are suitable for students at all levels such as building demo nano characterization tools that involve both hardware design and electronics development. The demo projects provide a bridge for the students to participate in the proposed research activities. Under-represented groups will be particularly encouraged and recruited in the education programs.Technical description:Atomic layered materials provide a rich platform to study topological physics in two dimensions. This project investigates several families of novel 2D materials and their heterostructures to characterize their topological states and explore new methods to manipulate and engineer their topological properties. The first material system is the recently discovered topological magnet, MnBi2Te4, which combines magnetism and topological order in one material and is a Chern insulator in few-layer samples. The proposed research aims to characterize key properties in few-layer samples that are important for the topological order, including the topological gap and the magnetic anisotropy. The second material system is graphene based moiré heterostructures, including twisted bilayer graphene aligned on hBN and graphene interfaced with moiré heterostructures of transition metal dichalcogenides. The formation of moiré superlattice with a large periodicity is expected to modulate the Landau level structures in graphene, which could potentially be used to tune the topological states in graphene. The third material system is heterostructures of 2D magnets. The proposed research explores different material combinations to introduce strong spin orbit coupling and break inversion symmetry at the interface which could induce the Dzyaloshinskii–Moriya interaction and create skyrmions in the magnetic materials. This project employs several scanning probe microscopy techniques, including microwave impedance microscopy (MIM), electrostatic force microscopy (EFM), and magnetic force microscopy (MFM), to probe the local electronic and magnetic properties and image the topological states in real space. The local probes provide complementary information to conventional transport techniques, which can bring in new insights on topological physics in two dimensions from a different perspective. Results from this research will provide important feedback for theorists to refine model parameters and for material scientist to optimize and improve sample qualities.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.