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arXiv:2504.18480 · Chinese Physics Letters

Nature of the 1/3 Magnetization Plateau in Spin-1/2 Kagome Antiferromagnets

Li-Wei He, Xinzhe Wang, Shun-Li Yu, Jian‐Xin Li

Published Jul 15, 2025• 3 citations
arXivHTMLPDF

Abstract

Abstract We investigate the origin of the 1/3 magnetization plateau in the S = 1/2 kagome antiferromagnetic Heisenberg model using the variational Monte Carlo and exact diagonalization methods, to account for the recent experimental observations in YCu 3 (OH) 6+ x Br 3− x and YCu 3 (OD) 6+ x Br 3− x . We identify three degenerate valence-bond-solid (VBS) states forming a 3 × 3 unit cell. These states exhibit David-star patterns in the spin moment distribution with only two fractional values −1/3 and 2/3, and are related through translational transformations. While the spin correlations in these VBS states are found to be short-range, resembling a quantum spin liquid, we show that they have a vanishing topological entanglement entropy and thus are topologically trivial many-body states. Our theoretical results provide strong evidence that the 1/3 magnetization plateau observed in recent experiments arises from these 3 × 3 VBS states with fractional spin moments.

DOI
10.1088/0256-307x/42/9/090704

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Abstract We investigate the origin of the 1/3 magnetization plateau in the S = 1/2 kagome antiferromagnetic Heisenberg model using the variational Monte Carlo and exact diagonalization methods, to account for the recent experimental observations in YCu 3 (OH) 6+ x Br 3− x and YCu 3 (OD) 6+ x Br 3− x . We identify three degenerate valence-bond-solid (VBS) states forming a 3 × 3 unit cell. These states exhibit David-star patterns in the spin moment distribution with only two fractional values −1/3 and 2/3, and are related through translational transformations. While the spin correlations in these VBS states are found to be short-range, resembling a quantum spin liquid, we show that they have a vanishing topological entanglement entropy and thus are topologically trivial many-body states. Our theoretical results provide strong evidence that the 1/3 magnetization plateau observed in recent experiments arises from these 3 × 3 VBS states with fractional spin moments.
The nature of the 1/9-magnetization plateau of the spin-12\frac{1}{2}21​ kagome Heisenberg antiferromagnet remains controversial due to the exotic physical properties and high complexity induced by the geometrical frustration. Instead of a Z3{\mathbb{Z}}_{3}Z3​ quantum spin liquid revealed on a cylinder, we show on an infinite-size lattice that the 1/9 plateau can be described by a valence bond crystal (VBC) that breaks spatial translational invariance. Consistent results are achieved by two accurate tensor network methods, namely the full-update infinite projected-entangled pair states and the projected-entangled simplex states. The VBC exhibits an hourglass pattern with $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ spatial symmetry, demonstrated by magnetizations, bond energies, and three-body correlators. The spatial inversion symmetry in the $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ VBC is instantly broken with the presence of the difference between the coupling strengths in the up and down triangles, suggesting the existence of gapless excitations. The gapless nature of the 1/9 plateau is further indicated by the scaling behaviors of the entanglement entropy and the correlation length, which indicate a c=1c=1c=1 conformal field theory.
We analyze the magnetization curve of the spin-1/2 kagome Heisenberg model in a magnetic field. Using state-of-the-art variational wave functions based on neural networks, we confirm the presence of robust magnetization plateaus at m=1/3m=1/3m=1/3, 5/95/95/9 and 7/97/97/9 of the saturation value, stabilized by a spontaneous symmetry breaking of lattice translations with a 3×3\sqrt{3}\times \sqrt{3}3​×3​ unit cell. Regarding the more challenging m=1/9m=1/9m=1/9 plateau, we find two competing valence bond crystals depending on the system size, both breaking translation as well as point group symmetries and with a larger 3×33\times 33×3 unit cell. Such quantum states with local modulations of the magnetization average values could be observed experimentally in the near future.
We present numerical evidence for the existence of an extended valence bond solid (VBS) phase at T=0T=0T=0 in the kagome S=1/2S=1/2S=1/2 Heisenberg antiferromagnet with ferromagnetic further-neighbor interactions. The VBS is located at the boundary between two magnetically ordered regions and extends close to the nearest-neighbor Heisenberg point. It exhibits a diamondlike singlet covering pattern with a 12-site unit cell. Our results suggest the possibility of a direct, possibly continuous, quantum phase transition from the neighboring q=0\mathbf{q}=0q=0 coplanar magnetically ordered phase into the VBS phase. Moreover, a second phase which breaks lattice symmetries, and is of likely spin-nematic type, is found close to the transition to the ferromagnetic phase. The results have been obtained using large-scale numerical exact diagonalization. We discuss implications of our results on the nature of the nearest-neighbor Heisenberg antiferromagnet.
We perform an exact-diagonalization study of the spin-12\frac{1}{2}21​ XXZ Heisenberg antiferromagnet on the kagome lattice at finite magnetization m=23m=\frac{2}{3}m=32​ with an emphasis on the XYXYXY point (Jz=0)({J}_{z}=0)(Jz​=0) and in the presence of a small chiral term. Recent analytic work by Kumar et al. [K. Kumar, K. Sun, and E. Fradkin, Phys. Rev. B 90, 174409 (2014)] on the same model, using a newly developed flux attachment transformation, predicts a plateau at this value of the magnetization described by a chiral spin liquid (CSL) with a spin Hall conductance of ${\ensuremath{\sigma}}_{xy}=\frac{1}{2}$. Such a state is topological in nature, has a ground-state degeneracy, and exhibits fractional excitations. We analyze the degeneracy structure in the low-energy manifold, identify the candidate topological states, and use them to compute the modular matrices and Chern numbers, all of which strongly agree with expected theoretical behavior for the ${\ensuremath{\sigma}}_{xy}=\frac{1}{2}$ CSL. In the limit of zero chirality, we find on most (not all) clusters that the topological invariants are still those of a CSL.
The spin-1/2 quantum antiferromagnet on the kagome lattice provides a quintessential example in strongly correlated electron physics in which the effects of both geometric frustration and quantum fluctuation are pushed to their limit. Among possible nonmagnetic ground states, the valence bond solid (VBS) with a 36-site unit cell is one of the most promising candidates. A natural theoretical framework for the analysis of such VBS order is to consider quantum states on a bond connecting the nearest-neighboring sites as fundamental quantum modes of the system and treat them as effectively independent ``bond particles.'' While correctly describing the VBS order in the ground state, this approach, known as the bond operator theory, significantly overestimates the lowest spin excitation energy. To overcome this problem, we take a next logical step in this paper to improve the bond operator theory and consider extended spin clusters as fundamental building blocks of the system. Depending on two possible configurations of the VBS order, various spin clusters are considered: (i) in the VBS order with staggered hexagonal resonance, we consider one spin cluster for a star of David shape and two spin clusters with each composed of a perfect hexagon and three attached dimers, and (ii) in the VBS order with uniform hexagonal resonance, we consider one spin cluster composed of a star of David and three attached dimers. It is shown that the majority of low-energy spin excitations are nearly or perfectly flat in energy. With most of its weight coming from the star of David, the lowest spin excitation has a gap much lower than the previous value obtained by the bond operator theory, narrowing the difference against exact diagonalization results. Despite the better agreement, however, the fact that the lowest spin excitation mostly comes from the star-of-David structure is at odds with the bond operator theory as well as previous series expansion studies. Expecting that the true low-energy spin excitations have contributions from both the star of David and its surroundings, we propose using two superclusters of spin in future work, treating spin excitations from both contributions on a more equal footing.
The spin-12\frac{1}{2}21​ Heisenberg model on a kagome lattice is a typical frustrated quantum spin system. The basic structure of a kagome lattice is also present in the kagome-strip lattice in one dimension, where a similar type of frustration is expected. We thus study the magnetization plateaus of the spin-12\frac{1}{2}21​ Heisenberg model on a kagome-strip chain with three-independent antiferromagnetic exchange interactions using the density-matrix renormalization-group method. In a certain range of exchange parameters, we find twelve kinds of magnetization plateaus, nine of which have magnetic structures breaking translational and/or reflection symmetry spontaneously. The structures are classified by an array of five-site unit cells with specific bond-spin correlations. In a case with a nontrivial plateau, namely a 3/10 plateau, we find long-period magnetic structure with a period of four unit cells.