Tailoring and probing electronic/magnetic structure of engineered magnetic topological insulators

非技术摘要：拓扑材料的显着特性，例如传导电流而不耗散的能力，在四十年前首次被发现，但仅在接近绝对零度温度和高磁场的极端实验条件下。后来人们认识到，这种显着的传输性能是来自材料的“拓扑结构”。这一认识开启了拓扑量子材料的时代，它有可能在没有外部磁场的情况下在更高的温度下实现显着的特性。 然而，在将这些技术承诺变为现实之前，需要克服一些技术挑战。 该研究计划旨在解决一些最重要的材料科学问题，并为使用多层异质结构定制新型磁性拓扑绝缘体奠定基础。 在教育方面，PI将在现有课程之外创建一门新课程，为学生提供学术培训，以便他们为进入拓扑量子材料这一令人兴奋的新研究领域做好充分准备。 技术摘要：在磁性拓扑绝缘体（MTI）中，磁性的引入打破了时间反演对称性，并在拓扑绝缘体（TI）的无质量表面态中产生了狄拉克质量隙。 在MTI显着的输运性质，如量子反常霍尔效应（QAHE）已被预测和观察到的非本征MTI材料（即TI与磁性掺杂剂）。然而，由于掺杂剂的无序效应，QAHE只能在非常低的温度（~ 30 mK）下观察到。新出现的本征MTI材料如MnBi 2 Te 4（MBT）通过将化学计量磁性层（MnTe）并入Bi 2 Te 3的中心提供了替代平台。 一个理想的本征MTI将是免费的掺杂无序，从而提供了可能性，观察QAHE的磁转变温度。 已经有几个研究小组报道了在~ 1 K下观察到QAHE，明显高于外在MTI，尽管仍然小于磁转变温度（~ 20 K）。 这些早期的工作刺激了世界范围内的密集研究工作。 然而，若干未决问题仍未解决。 与此同时，人们致力于设计新的磁性拓扑量子材料，而不仅仅是简单的MBT及其相关化合物。 本计画结合分子束磊晶与原位扫描探针显微镜技术，研究人工制造的MTI与MTI/TI异质结构。 目标有三个方面：（a）通过最终控制缺陷密度和化学势，逐层控制MTI和MTI/TI异质结构的形成;（B）解决挑战当前对拓扑表面状态和磁性织构之间的联系的理解的关键的突出问题;（c）确定使用MTI/TI超晶格的拓扑性质的人工工程的关键设计参数。 在教育上，在这个研究计划中培养的研究生获得了广泛的科学视野。 通过拓扑量子材料的特殊课程的设计，PI将显着拓宽当代凝聚态材料的研究生/本科教育。该奖项反映了NSF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: Remarkable properties of topological materials, such as ability to conduct electric current without dissipation, were first discovered four decades ago, but only under extreme experimental conditions of nearly absolute zero temperature and high magnetic field. It was subsequently recognized that such a remarkable transport property is derived from material “topology”. This recognition has launched the era of topological quantum materials with the potential to realize the remarkable properties at a much higher temperature without an external magnetic field. There are, however, several technical challenges that need to be overcome before bringing such technical promises to reality. This research program is set up to address some of the most important materials science issues and to lay the foundations for tailoring the new class of magnetic topological insulators using multi-layered heterostructures. Educationally, the PI will create a new course beyond the current curriculum to provide students with academic training so they can be well-prepared to enter this new exciting research field of topological quantum materials. Technical Abstract: In magnetic topological insulators (MTI), the incorporation of magnetism breaks the time reversal symmetry and creates a Dirac mass gap in the otherwise massless surface states of topological insulators (TI). In MTI remarkable transport properties such as quantum anomalous Hall effect (QAHE) have been predicted and observed in extrinsic MTI materials (i.e. TI with magnetic dopants). However, due to the dopant disorder effect, the QAHE can be observed only at a very low temperature (~ 30 mK). The newly emerged intrinsic MTI materials such as MnBi2Te4 (MBT) offers an alternative platform by incorporating a stoichiometric magnetic layer (MnTe) into the center of Bi2Te3. An ideal intrinsic MTI would be free of dopant disorder, thus offering the possibility to observe QAHE up to the magnetic transition temperature. Already several groups have reported observation of QAHE at ~ 1K, significantly higher than that in extrinsic MTI, albeit still smaller than the magnetic transition temperature (~ 20K). These earlier works have stimulated intensive worldwide research work. However, several outstanding issues remain unresolved. Meanwhile, efforts have been devoted to designing new magnetic topological quantum materials beyond simple MBT and related compounds. This project combines molecular beam epitaxy with in-situ scanning probe microscopy to study artificially engineered MTI and MTI/TI heterostructures. The object is three-fold: (a) controlling the formation of MTI and MTI/TI heterostructure layer-by-layer with ultimate control in defect density and chemical potential; (b) resolving key outstanding issues that challenge the current understanding of the connection between the topological surface states and the magnetic textures; (c) determining key designing parameters for artificial engineering of topological properties using MTI/TI superlattices. Educationally, the graduate students trained in this research program gain a broad scientific perspective. Through the design of a special course in topological quantum materials, the PI will significantly broaden graduate/undergraduate education in contemporary condensed matter physic. The program also broadens the participation of underrepresented groups.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.