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

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 YCu₃(OH)_6+xBr_3-x and YCu₃(OD)_6+xBr_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.