Exploration of New Magnetic Quantum Materials
Restricted (Penn State Only)
- Author:
- Guan, Yingdong
- Graduate Program:
- Physics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 16, 2024
- Committee Members:
- Venkatraman Gopalan, Outside Unit & Field Member
Nitin Samarth, Major Field Member
Jun Zhu, Major Field Member
Zhiqiang Mao, Chair & Dissertation Advisor
Mauricio Terrones, Program Head/Chair - Keywords:
- Bi2Se3
Weyl semimetal
Magnetic topological insulator
Ferrovalley material
Single crystal - Abstract:
- Topological quantum states have emerged as a transformative class of materials within condensed matter physics, profoundly extending its traditional boundaries. The revolutionary concept of topological order, first conceptualized through the exploration of the quantum Hall effect, represents a significant departure from Landau's symmetry-breaking paradigm. This new classification system divides materials into topologically trivial and non-trivial categories based on their electronic properties in momentum space. This shift has led to the discovery of diverse novel topological quantum states, such as quantum Hall states, quantum anomalous Hall states, topological insulators (TI), Weyl and Dirac semimetals, and axion insulators. The search for new topological quantum states, the realization of theoretical predictions, and the potential for spintronic applications drive the need for innovative material platforms. Often, these exotic topological states emerge from the complex interplay between non-trivial topology and other physical phenomena, including superconductivity and magnetism. This dissertation is dedicated to exploring new magnetic quantum materials that exhibit intricate interactions between non-trivial topology and magnetism, which are pivotal for advancing our understanding and application of topological phenomena. Our research centers around four material systems, each with distinctive magnetic and topological properties: the intrinsic magnetic topological insulator Mn(Bi,Sb)4Te7, the potential ferrovalley candidate Cr0.32Ga0.68Te2.33, the well-established topological insulator Bi2Se3, and SnMnBi2Te5, notable for its high Curie temperature ferromagnetism and prospective non-trivial topological states. In Chapters 3 and 4, we delve into the nuanced modulation of chemical potential and magnetic properties in the Mn(Bi,Sb)4Te7 system facilitated by strategic Sb doping. Our experimental findings in Mn(Bi,Sb)4Te7 single crystals, developed through melt growth techniques and displaying ferromagnetism at low Sb concentrations, point to the emergence of a Weyl semimetal phase. This phase is detected near the charge neutrality point through meticulous electrical transport measurements, suggesting a profound linkage between magnetism and electronic topology in this system. Chapter 5 details our discovery and characterization of Cr0.32Ga0.68Te2.33, a material that exhibits ferromagnetic properties below 20 K and demonstrates significant spin splitting in its band structure as predicted by Density Functional Theory (DFT) calculations. The combination of ferromagnetism and clear spin polarization earmarks this material as a potent candidate for ferrovalley applications, a novel and promising area of spintronics. In Chapter 6, we discuss the use of a new crystal growth technique - double crucible vertical Bridgman (DCVB) furnace to grow Bi2Se3 single crystals. Bi2Se3 is a renowned topological insulator, but its application has been historically hindered by a high defect density. By employing the DCVB method, we have significantly reduced its defect densities, enhancing the material's suitability for advanced spintronic devices. This advancement paves the way for using Bi2Se3 in practical high-performance applications where low defect densities are crucial. Lastly, Chapter 7 explores SnMnBi2Te5, derived from the parent compounds Sn2Bi2Te5 and Mn2Bi2Te5. This material shows a Curie temperature of 50 K, well above the Néel temperature of MnBi2Te4. Its resistivity also exhibits an upturn below the Curie temperature, signaling its non-trivial topological character. This compound offers an new opportunity for probing exotic quantum states arising from the interplay between magnetism and topology. The comprehensive investigations presented in this dissertation not only enrich our comprehension of the fundamental aspects of topological materials but also establish a robust foundation for their future technological applications, particularly in the domain of high-temperature spintronics. This work underscores the pivotal role of innovative material synthesis and characterization in exploring topological quantum states.