過去のセミナー

<10/2024>
Date
10月4日(金) 14:00-
Speaker
花井 亮(京都大学基礎物理学研究所)
Title
金属強磁性体における光誘起非相反相転移
<6/2024>
Date
6月6日(木) 13:50-
Speaker
Ayushi Singhania (Okinawa Institue of Science and Technology)
Title
Emergence of vortex state in the S = 1 Kitaev-Heisenberg model with single-ion anisotropy

The search for Kitaev spin liquid states has recently broadened to include a number of honeycomb materials with integer spin moments. The qualitative difference with their spin-1/2 counterparts is the presence of single- ion anisotropy (SIA). This motivates our investigation of the effects of SIA on the ground state of the spin-1 Kitaev-Heisenberg (KH) model using the density-matrix renormalization group which allows construction of detailed phase diagrams around the Kitaev points. We demonstrate that positive out-of-plane SIA induces an in-plane vortex state without the need for off-diagonal interactions. Conversely, negative SIA facilitates the emergence of a ferromagnetic state in presence of antiferromagnetic Heisenberg interactions, while a N ?eel state can emerge for ferromagnetic Heisenberg coupling. These findings, pertinent even for weak SIA, not only enhance our theoretical understanding of the spin-1 KH model but also propose experimental prospects for observing these novel magnetic states in material realizations.


<5/2024>
Date
5月28日(火) 10:30-
Speaker
大西 由吾(Condensed Matter Theory group, MIT Physics)
Title
Fundamental gap bound in insulators

Insulating states with a finite energy gap are ubiquitous. The finite energy gap between the ground state and the first excited state results in vanishing DC conductivity at zero temperature, i.e., insulating states. Much research effort has been put into exploring interesting insulating ground state properties, including topological responses such as the integer/fractional quantum Hall effect. On the other hand, the size of the energy gap seems not to have been paid much attention so far, even though realizing a large energy gap is crucial to experimentally observe these interesting ground state properties. In this talk, I will present two recent results on fundamental gap bounds in insulators [1,2]. The first one is a gap bound in terms of the dielectric constant, while the second bound is a gap bound in terms of the Chern number for integer/fractional Chern insulators. Both bounds are applicable to any gapped insulators, including strongly correlated systems. Our results reveal a relation between the ground state properties and the energy gap, suggesting a new research direction.

[1] Yugo Onishi, Liang Fu, Fundamental bound on topological gap. arXiv:2306.00078

[2] Yugo Onishi, Liang Fu, Universal relation between energy gap and dielectric constant. arXiv:2401.04180


<4/2024>
Date
4月4日(木) 10:30-
Speaker
藤本 大仁(東京大学工学研究科物理工学専攻 森本研究室)
Title
Higher vortexability: zero field realization of higher Landau levels

The rise of moir? materials has led to experimental realizations of integer and fractional Chern insulators in small[1] or vanishing magnetic fields[2]. At the same time, a set of minimal conditions sufficient to guarantee an Abelian fractional state in a flat band was identified, namely "ideal" or "vortexable" quantum geometry[3,4]. Such vortexable bands share essential features with the lowest Landau level, while excluding the need for more fine-tuned aspects such as flat Berry curvature. A natural and important generalization is to ask if such conditions can be extended to capture the quantum geometry of higher Landau levels, particularly the first (1LL), where non-Abelian states at ν = 1/2, 2/5 are known to be competitive[5]. The possibility of realizing these states at zero magnetic field, and perhaps even more exotic ones, could become a reality if we could identify the essential structure of the 1LL in Chern bands. In this work[6], we introduce a precise definition of 1LL quantum geometry, along with a figure of merit that measures how closely a given band approaches the 1LL. We apply the definition to identify two models with 1LL structure ? a toy model of double bilayer twisted graphene and a more realistic model of strained Bernal graphene.

[1] Y. Xie, A. T. Pierce, J. M. Park, D. E. Parker, E. Khalaf, P. Ledwith, Y. Cao, S. H. Lee, S. Chen, P. R. Forrester, et al., Fractional Chern insulators in magic-angle twisted bilayer graphene, Nature 600, 439 (2021).

[2] J. Cai, E. Anderson, C. Wang, X. Zhang, X. Liu, W. Holtzmann, Y. Zhang, F. Fan, T. Taniguchi, K. Watanabe, et al., Signatures of fractional quantum anomalous hall states in twisted MoTe2, Nature 622, 63 (2023).

[3] J. Wang, J. Cano, A. J. Millis, Z. Liu, and B. Yang, Exact landau level description of geometry and interaction in a flatband, Physical review letters 127, 246403 (2021).

[4] P. J. Ledwith, A. Vishwanath, and D. E. Parker, Vortexability: A unifying criterion for ideal fractional Chern insulators, Physical Review B 108, 205144 (2023).

[5] G. Moore and N. Read, Nonabelions in the fractional quantum hall effect, Nuclear Physics B 360, 362 (1991).

[6] Manato Fujimoto, Daniel E. Parker, Junkai Dong, Eslam Khalaf, Ashvin Vishwanath, and Patrick Ledwith, Higher vortexability: zero field realization of higher Landau levels, arXiv preprint arXiv:2403.00856 (2024).


<1/2024>
Date
1月15日(月) 14:00-
Speaker
戎 弘実(京都大学基礎物理学研究所)
Title
Foliated BF theory and multipole symmetry

分数量子ホール効果、スピン液体などのTopological ordered phase は主に 2 次元系に現れる新奇な相で、これらの相の重要な特徴は分数電荷の励起、すなわち anyon が存在することです。従来のanyonは励起されるとバルクのどこでも動くことができますが、最近になりfracton topological phaseと呼ばれる、この性質を満たさないtopological相が理論的に提案されました。この相では分数電荷の励起の動く範囲に制限がかかり、系の縮退度も局所的な情報によってしまいます。この性質から従来のtopologicalな場の量子論は構築できず、相を記述する新しい理論が必要となっています。この背景のもと、新しい対称性であるmulti-pole symmetry(多極子の保存に対応する対称性)に着目し、そのゲージ理論から新しいtopologicalな場の理論を構築することを議論します。