UCLA

Topological insulator (TI) is a new class of quantum material with inverted band structure caused by strong spin-orbit coupling (SOC), resulting an insulating bulk and topologically protected surface states. By introducing magnetism to break the time-reversal symmetry, it can host emerging topological quantum phases such as quantum anomalous Hall (QAH) effect, axion insulator quantum phase, and can host chiral Majorana fermion when coupled to superconductors. The goal of this dissertation is to study these quantum phases by material growth and transport measurements.

First, we studied the growth condition by molecular beam epitaxy (MBE) and achieved high quality magnetically doped topological insulator (MTI) film growth that can host QAH effect. To understand the different origins of zero Hall conductance plateau in QAH insulators and axion insulators, we carried out the thickness dependent study on MTI sandwiched structures and observed a dimension crossover between 2D QAH and 3D axion insulators by minor loops measurements and scaling analysis. Then, we investigated the QAH insulator and antiferromagnetic heterostructures which offer an additional degree of freedom to manipulate the QAH state in MTI. By proximity coupling the QAH insulator to antiferromagnetic insulator Al-Cr2O3, we observed an exchange biased QAH effect, and the exchange bias can be effectively controlled by a field training process. In addition, chiral Majorana edge modes are reported to exist in QAH insulator and superconductors heterostructures. Aiming to understand the discrepancy of recent experimental reports, we fabricated QAH insulator with superconductor and normal metals hybrid devices, and we found the details of material characteristics, interface conditions play important roles on the experiment observations. Finally, two collaborative works closely related to the main scope of this dissertation are introduced. To probe the size limit and to understand the inter-channel scattering of QAH effect, we fabricated mesoscopic scale sub-micron size QAH insulator devices and carried out size dependent studies. Enabled by the successful growth of QAH film on insulating 2D van-der-Waals substrate MICA, we were able to fabricate dual-gate QAH device by exfoliation and transfer techniques. By applying a vertical electric field, we observe a topological phase transition from QAH to a trivial insulator state. This research will greatly improve the understanding of emerging topological quantum physics, and lead to the new revolution of quantum electronic devices.