| 発表者名 |
新井 敦郎 |
| 指導教員名 |
求 幸年 教授 |
| 発表題目（英語） |
Variational Monte Carlo Study of Quantum Spin Liquids and Their Nature |
| 要旨（英語） |
Quantum spin liquids (QSLs) represent a fascinating class of quantum phases that defy conventional magnetic ordering even at absolute zero temperature, and can be observed in frustrated systems. Characterized by the absence of symmetry breaking, QSLs exhibit highly entangled ground states with remarkable properties such as topological order and fractionalized excitations. However, the very definition of QSLs and their frustrated nature renders their theoretical and numerical analysis exceptionally challenging.
In the numerical study of QSLs, variational Monte Carlo (VMC) methods have been extensively employed. VMC allows for flexible construction of variational wavefunctions that can incorporate key physical insights, such as resonating valence bond (RVB) structures, and enables the calculation of ground state energies and correlation functions with controlled statistical errors. This approach is especially useful for large-scale systems where exact diagonalization or tensor network methods become impractical.
Despite its success and sophistication in providing candidate ground states for QSLs, VMC faces significant limitations when it comes to diagnosing the physical nature of the resulting wavefunctions. The goal of this research is to develop more interpretable variational wavefunctions that bridge the gap between mean-field theoretical frameworks and the numerically optimized VMC solutions. By constructing variational ansätze that retain physical transparency while remaining expressive enough to capture the essential features of QSLs, we aim to classify different QSL phases and extract their topological properties directly from the wavefunction structure. |
| 発表言語 |
日本語 |
|
| 発表者名 |
池上 草玄 |
| 指導教員名 |
求 幸年 教授 |
| 発表題目（英語） |
Multipole orders and quantum spin liquids in spin-3/2 honeycomb models |
| 要旨（英語） |
Quantum spin liquids have been extensively studied for their remarkable properties, such as the absence of magnetic order, fractionalized excitations, and topological order [1, 2]. While most research has focused on S=1/2 systems, there have been several efforts to explore magnets with S ≥ 1. These magnets exhibit additional multipolar degrees of freedom, offering the potential for a rich variety of both ordered and disordered states with multipolar character. Indeed, in S = 1 systems with quadrupolar degrees of freedom, diverse quantum states have been realized in models with bilinear, biquadratic, and anisotropic interactions [3]. On the other hand, S = 3/2 systems additionally possess octupolar degrees of freedom and can host two different exotic states, the Kitaev spin liquid [4, 5] and the Affleck-Kennedy-Lieb-Tasaki (AKLT) state [6, 7] on a honeycomb lattice, suggesting the potential for much richer quantum phenomena. However, the effects of the biquadratic and bicubic interactions in S = 3/2 systems have not been fully clarified, and the interplay between quantum spin liquids and multipole orders remains elusive.
In this study, we investigate the ground states of two different S=3/2 quantum spin models on a honeycomb lattice. One is the “b3 model” with isotropic bilinear, biquadratic, and bicubic interactions, and the other is the Kitaev-AKLT model, which is a combination of two exactly solvable models that stabilize quantum spin liquid ground states. Using a semiclassical approach based on SU(4) spin coherent states, we show that the b3 model exhibits multipolar
ordered phases with suppressed magnetic dipole moments and that the Kitaev-AKLT model exhibits various dipolar ordered phases, including noncoplanar multiple-Q states.
[1] L. Savary and L. Balents, Rep. Progr. Phys. 80, 016502 (2016).
[2] Y. Zhou, K. Kanoda, and T.-K. Ng, Rev. Mod. Phys. 89, 025003 (2017).
[3] R. Pohle, N. Shannon, and Y. Motome, Phys. Rev. B 107, L140403 (2023).
[4] A. Kitaev, Ann. Phys. 321, 2 (2006).
[5] H.-K Jin, W. M. H. Natori, F. Pollmann, and J. Knolle, Nat. Commun. 13, 3813 (2022). [6] I. Affleck, T. Kennedy, E. H. Lieb, and H. Tasaki, Phys. Rev. Lett. 59, 799 (1987).
[7] I. Affleck, T. Kennedy, E. H. Lieb, and H. Tasaki, Commun. Math. Phys. 115, 477 (1988). |
| 発表言語 |
日本語 |
|
| 発表者名 |
LAURIENZO Joseph Thomas |
| 指導教員名 |
求 幸年 教授 |
| 発表題目（英語） |
Theoretical Study of Quantum Fluctuations in Topological Spin Textures |
| 要旨（英語） |
Topological spin textures, such as two-dimensional skyrmions and three-dimensional hopfions, are a trending front of research due to their novel physical properties stemming from the topological robustness of their particle-like nature and quantum geometric effects due to noncollinear and noncoplanar spin configurations. They have been extensively investigated using classical spin models and semi-classical approaches for quantum spins. However, effects of quantum fluctuations on these spin textures can be crucial, e.g., in the formation of their crystals and real-time dynamics, especially in low-dimensional systems. Such intriguing physics remains largely unexplored, primarily due to the lack of a suitable theoretical framework capable of adequately addressing quantum fluctuations.
The aim of this study is to elucidate the effects of quantum fluctuations on topological spin textures. By employing advanced theoretical techniques, such as the density matrix renormalization group (DMRG) and variational Monte Carlo methods, we will investigate quantum phenomena, such as melting behavior of crystals of topological spin textures and their real-time dynamics. As an initial step, we will discuss the case of one-dimensional topological spin textures, called the chiral solitons, in monoaxial chiral magnets. |
| 発表言語 |
英語 |
|
