QUANTUM TRANSPORT IN TOPOLOGICAL MATERIALS AND PROXIMITY EFFECT IN FERROMAGNETIC NANOWIRES
Open Access
- Author:
- Jiang, Jue
- Graduate Program:
- Physics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 29, 2019
- Committee Members:
- Moses H. W. Chan, Dissertation Advisor/Co-Advisor
Moses H. W. Chan, Committee Chair/Co-Chair
Cui-zu Chang, Committee Member
Chaoxing Liu, Committee Member
Roman Engel-Herbert, Outside Member - Keywords:
- Quantum anomalous Hall effect
Topological insulator
Physical vapor deposition
Dilution refrigerator
Nanolithography
Heterostructures - Abstract:
- The magnetic topological insulator (TI) and the superconductor look different from each other, however, they share a similar electrical transport property of a profound significance: zero resistance. Their potential in the future low-power-consumption applications is beyond measure, therefore, the research attention on TI has been dramatically expanding since its debut in 2009, and the study of superconductivity keeps inspiring people of generations in the past 100 years. The realization of the non-dissipative channel in magnetic TI requires the broken time-reversal-symmetry by ferromagnetic dopants. The engineering of ferromagnetism, in turn, induces new topological phenomena. In this dissertation, we show that by fabricating a magnetic TI/pure TI/magnetic TI sandwich structure, rigorous quantum anomalous Hall (QAH) effect could be realized along with ‘axion insulator’ state or topological Hall effect, depending on the sample structure. In Cr-doped/non-doped/V-doped TI heterostructures, QAH effect emerges when the magnetizations of the Cr-doped and V-doped magnetic layers are parallel, while an ‘axion insulator’ state with zero Hall resistance and insulating longitudinal resistance appears when magnetization alignment is anti-parallel; In an Cr-doped/non-doped/Cr-doped TI structure, by tuning the chemical potential, QAH effect crossovers to topological Hall effect, where the electron spins form topologically non-trivial spin textures. Superconductivity, on the other hand, would be destroyed in a ferromagnet due to the decoupling of a Cooper pair by the exchange coupling. Therefore, a spin-singlet Cooper pair is not able to survive in a ferromagnet more than a few nanometers. In this dissertation, however, we show that in a ferromagnetic Ni nanowire (500 nm wide and 40 nm thick), by simply adding a thin Cu buffer layer with natural oxidation between superconducting/ferromagnetic interface, an unusual long-range superconducting proximity effect (up to 136 nm) emerges. Strong evidence points to the Cu oxides for providing a noncollinear magnetic profile that is crucial to the induction of spin-triplet. The spin-triplet pairing can have two electrons with the same spin direction, and thus immune to the exchange coupling of the ferromagnetic nanowire. The experiments of magnetic TI heterostructures and spin-triplet superconductivity presented in this dissertation would inspire more relating studies and pave the way for next-generation energy-efficient spintronic and electronic applications.