S=1磁性体における磁気励起

• Material Type: 博士論文
• Title: S=1磁性体における磁気励起
• Author/Editor: Remund, Kimberly
• Author Heading: Remund, Kimberly
• Publication Date: 2022-11-30
• Publication Date (W3CDTF): 2022-11-30
• Alternative Title: Spin–1 Magnets and Their Excitations
• Degree grantor/type: Okinawa Institute of Science and Technology Graduate University
• Date Granted: 2022-11-30
• Date Granted (W3CDTF): 2022-11-30
• Dissertation Number: 甲第112号
• Degree Type: Doctor of Philosophy
• Text Language Code: eng
• Target Audience: 一般
• Note (General): Nature sometimes arranges itself in extremely curious ways, sowing the seed of very intriguing physics. Magnetic systems offer a rich variety of interesting features. They are traditionally studied in either their classical (S ! 1), or their extreme quantum limit (S = 12). However, magnetic degrees of freedom in spin systems span within a whole spectrum range and do not necessarily reduce to the specific case found at the extremities. Spin–1 magnets provide a good example of what happens to ground state and excitations properties for such instance. Indeed, a spin–1 is special, in the sense that, besides displaying dipolar degrees of freedom, a spin–1 can also exhibit on-site quadrupolar degrees of freedom, while retaining its quantum characteristics. Therefore, spin–1 systems are often used as examples to refer to spin-nematic order in magnetic insulators, Fe-based superconductors, or cold atoms. Unlike for spin–1 2, which in the classical limit can be represented by an O(3) vector, for spin–1, an O(3) vector does not completely describe all of what a spin–1 can do, namely intrinsically exhibiting quadrupoles. In this Thesis, I develop a united framework that enables us to treat dipolar and quadrupolar degrees of freedom of a spin–1 moment on an equal footing. My method is based on the extension of the usual su(3) algebra describing a quantum spin–1 into the u(3) algebra. Within the u(3) formalism, I derive equations of motion (EoM) for the objects living in the u(3) algebra. The u(3) approach enables the appropriate formulation for both classical and quantum derivations. Moreover, the EoM take a simple form suitable for numerical implementation. I illustrate this method by applying it to the well-known Bilinear-Biquadratic model on the triangular lattice for the ferroquadupolar state. This study is supported through classical low-temperature expansion in order to probe the thermodynamical properties, as well as quantum multi-bosons theory that allows to access dynamics. These results are validated by comparison with numerical simulations classical Monte Carlo (MC) and Molecular Dynamics (MD) respectively, both expressed in terms of u(3) objects. I show that at sufficiently low temperature numerical simulations can be corrected for the classical statistics, and the fully quantum zero-temperature analytical results are retrieved. Additionally, I confirm that our method is also applicable to anisotropic models, which is of experimental relevance. Finally, some new ideas, including the description of topological defects in spin–1 magnets and the generalization of the commonly used Self-Consistent Gaussian Approximation to the degrees of freedom of a spin–1 expressed within our u(3) framework are explored.
• DOI: 10.15102/1394.00002589; https://doi.org/10.15102/1394.00002589
• Persistent ID (NDL): info:ndljp/pid/12651517; https://dl.ndl.go.jp/pid/12651517
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• Collection (particular): 国立国会図書館デジタルコレクション > デジタル化資料 > 博士論文; https://dl.ndl.go.jp/collections/A00014
• Acquisition Basis: 博士論文（自動収集）
• Date Accepted (W3CDTF): 2023-03-02T11:18:10+09:00
• Format (IMT): application/pdf
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• Periodical Title (URI): http://dx.doi.org/10.15102/1394.00002589; https://oist.repo.nii.ac.jp/records/2864
• Data Provider (Database): 国立国会図書館 : 国立国会図書館デジタルコレクション; https://dl.ndl.go.jp