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

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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.