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Tuning Magnetism and Superconductivity in Topological Material Candidates.

Record Type:	Language materials, manuscript : Monograph/item
Title/Author:	Tuning Magnetism and Superconductivity in Topological Material Candidates./
Author:	Qian, Tiema.
Description:	1 online resource (163 pages)
Notes:	Source: Dissertations Abstracts International, Volume: 85-12, Section: B.
Contained By:	Dissertations Abstracts International85-12B.
Subject:	Condensed matter physics. -
Online resource:	click for full text (PQDT)
ISBN:	9798382788708

Tuning Magnetism and Superconductivity in Topological Material Candidates.
Qian, Tiema.

Tuning Magnetism and Superconductivity in Topological Material Candidates. - 1 online resource (163 pages)

Source: Dissertations Abstracts International, Volume: 85-12, Section: B.

Thesis (Ph.D.)--University of California, Los Angeles, 2024.

Includes bibliographical references

Since the theoretical proposals of topological phases of matter and topological phase transitions, the experimental realization of topological materials and associated emerging phenomena had become an essential goal in condensed matter physics. The experimental discoveries of the quantum Hall effect (QHE), the quantum spin Hall effect (QSHE), and three-dimensional (3D) time-reversal symmetry protected topological insulators (TIs) further sparked intensive research effort, leading to a kaleidoscope of topological phases and realizations of diverse topological materials, such as Dirac semimetals, Weyl semimetals, magnetic topological insulators, topological superconductor, etc. A topological phase of matter is distinguished from trivial materials by showing a nonzero topological invariant and topologically protected surface states, which result in exotic phenomena in its transport, thermodynamic, optical and other physical properties. Practically, new topological phases may be realized by combining the topological band structure with other physical aspects. For example, breaking time reversal symmetry in an existing TI by introducing ferromagnetism or net magnetization, a gaped surface state with dissipationless edge conduction may emerge, resulting in the quantum anomalous Hall effect (QAHE) in the absence of external magnetic field.My thesis focuses on the study of topological materials with two major research themes. One is the synthesis, characterization and tuning of ternary Mn-Bi-Te magnetic topological insulators, including the synthetic exploration of new magnetic topological insulators, with a focus on the investigation of the interplay of magnetism and band topology through doping and external pressure. The other involves investigating proposed topological superconductor candidates through external stimuli, such as uniaxial strain and hydrostatic pressure, to enhance our understanding of superconductivity in such material systems.QAHE was first realized in magnetically doped TI Cr0.15(Bi0.1Sb0.9)0.85Te0.3 thin film in 2013. However, doped materials brought inevitable sample inhomogeneity, and thus the phenomenon was only observed at very low temperature, in the range of mK. To overcome this material challenge, it is believed that intrinsic magnetic TIs, i.e., stoichiometric magnetic TIs without doping, will be superior due to their higher magnetic and electronic homogeneity compared to doped materials. The first intrinsic magnetic TI MnBi2Te4 was discovered in 2018. It is an antiferromagnetic (AFM) TI with van der Wall (vdW) coupling that orders below 24 K. Its spins align ferromagnetically (FM) in individual planes but AFM between neighboring layers. Due to its vdW nature, it can be exfoliated and fabricated into oddlayer devices with net magnetization, theoretically proposed as QAH insulators, or into even-layer devices that preserve AFM, proposed as axion insulators. QAH effect was soon observed experimentally at 1.6 K and zero field in a 5-layer device with Hall signal plateau at 0.998h/e2 while Layer Hall effect and quantum metric nonlinear Hall effect were observed in 6-layer devices. To better engineer the magnetic properties of this family, growth trails had led to the discovery of new intrinsic magnetic TIs that with alternating [Bi2Te3] and magnetic [MnBi2Te4] layers, forming the natural heterostructural series of MnBi2nTe3n+2. In this family of compound, Mn layer is brought apart by adding more layers of Bi2Te3, causing the phase to eventually evolve from AFM TI in MnBi2Te4 to FM axion insulator in MnBi8Te13.Although field-induced quantized Hall conductance has been reported by a few groups in both odd- and even-layer MnBi2Te4 devices, there is only one report showing the observation of zero-field QAHE. Several major reasons why it remains challenging to realize QAH in this system: chemical disorders in the bulk samples; chemical disorders introduced during the device fabrication process; weak net magnetism in odd-layer devices. Synthesis efforts are needed to reduce the chemical disorders, particularly the MnBi antisites that are most detrimental to the realization of a universal surface gap and thus QAH, to improve the outcome while the weak net magnetism in devices can be addressed by achieving a ferromagnetic (FM) ground state in bulk sample. Mn(Bi1−xSbx)2Te4 was made with the hope that it might address the problems. The doping indeed induces FM ground state of the Mn sublattice. However, it also significantly increases the MnBi antisite concentration from around 2% to about 16%, forming a secondary FM Mn sublattice that aligns antiferromagnetically with the dominant Mn sublattice. As a result, the Hall conductance in devices made from Sb-doped samples is far from the quantization value. Therefore, progress in solving this outstanding material challenge remains unsatisfactory. The theme of my thesis work on the Mn-Bi-Te system focuses on addressing these issues by conducting doping trials to suppress MnBi antisites (chapter 3), investigating the competition between FM and AFM energy scales in the system (chapter 4), and searching for new magnetic topological insulators. (Abstract shortened by ProQuest).

Electronic reproduction.
Ann Arbor, Mich. :
ProQuest,
2024

Mode of access: World Wide Web

ISBN: 9798382788708Subjects--Topical Terms:

1148471
Condensed matter physics.
Subjects--Index Terms:

Crystal growthIndex Terms--Genre/Form:

554714
Electronic books.

Tuning Magnetism and Superconductivity in Topological Material Candidates.
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Practically, new topological phases may be realized by combining the topological band structure with other physical aspects. For example, breaking time reversal symmetry in an existing TI by introducing ferromagnetism or net magnetization, a gaped surface state with dissipationless edge conduction may emerge, resulting in the quantum anomalous Hall effect (QAHE) in the absence of external magnetic field.My thesis focuses on the study of topological materials with two major research themes. One is the synthesis, characterization and tuning of ternary Mn-Bi-Te magnetic topological insulators, including the synthetic exploration of new magnetic topological insulators, with a focus on the investigation of the interplay of magnetism and band topology through doping and external pressure. The other involves investigating proposed topological superconductor candidates through external stimuli, such as uniaxial strain and hydrostatic pressure, to enhance our understanding of superconductivity in such material systems.QAHE was first realized in magnetically doped TI Cr0.15(Bi0.1Sb0.9)0.85Te0.3 thin film in 2013. However, doped materials brought inevitable sample inhomogeneity, and thus the phenomenon was only observed at very low temperature, in the range of mK. To overcome this material challenge, it is believed that intrinsic magnetic TIs, i.e., stoichiometric magnetic TIs without doping, will be superior due to their higher magnetic and electronic homogeneity compared to doped materials. The first intrinsic magnetic TI MnBi2Te4 was discovered in 2018. It is an antiferromagnetic (AFM) TI with van der Wall (vdW) coupling that orders below 24 K. Its spins align ferromagnetically (FM) in individual planes but AFM between neighboring layers. Due to its vdW nature, it can be exfoliated and fabricated into oddlayer devices with net magnetization, theoretically proposed as QAH insulators, or into even-layer devices that preserve AFM, proposed as axion insulators. QAH effect was soon observed experimentally at 1.6 K and zero field in a 5-layer device with Hall signal plateau at 0.998h/e2 while Layer Hall effect and quantum metric nonlinear Hall effect were observed in 6-layer devices. To better engineer the magnetic properties of this family, growth trails had led to the discovery of new intrinsic magnetic TIs that with alternating [Bi2Te3] and magnetic [MnBi2Te4] layers, forming the natural heterostructural series of MnBi2nTe3n+2. In this family of compound, Mn layer is brought apart by adding more layers of Bi2Te3, causing the phase to eventually evolve from AFM TI in MnBi2Te4 to FM axion insulator in MnBi8Te13.Although field-induced quantized Hall conductance has been reported by a few groups in both odd- and even-layer MnBi2Te4 devices, there is only one report showing the observation of zero-field QAHE. Several major reasons why it remains challenging to realize QAH in this system: chemical disorders in the bulk samples; chemical disorders introduced during the device fabrication process; weak net magnetism in odd-layer devices. Synthesis efforts are needed to reduce the chemical disorders, particularly the MnBi antisites that are most detrimental to the realization of a universal surface gap and thus QAH, to improve the outcome while the weak net magnetism in devices can be addressed by achieving a ferromagnetic (FM) ground state in bulk sample. Mn(Bi1−xSbx)2Te4 was made with the hope that it might address the problems. The doping indeed induces FM ground state of the Mn sublattice. However, it also significantly increases the MnBi antisite concentration from around 2% to about 16%, forming a secondary FM Mn sublattice that aligns antiferromagnetically with the dominant Mn sublattice. As a result, the Hall conductance in devices made from Sb-doped samples is far from the quantization value. Therefore, progress in solving this outstanding material challenge remains unsatisfactory. The theme of my thesis work on the Mn-Bi-Te system focuses on addressing these issues by conducting doping trials to suppress MnBi antisites (chapter 3), investigating the competition between FM and AFM energy scales in the system (chapter 4), and searching for new magnetic topological insulators. (Abstract shortened by ProQuest).
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