Emergent Spin-Gapped Magnetization Plateaus in a Spin-1/2 Perfect Kagome Antiferromagnet.

The two-dimensional spin-1/2 kagome Heisenberg antiferromagnet is believed to host quantum spin liquid (QSL) states with no magnetic order, but its ground state remains largely elusive. An important outstanding question concerns the presence or absence of the 1/9 magnetization plateau, where exotic quantum states, including topological ones, are expected to emerge. Here we report the magnetization of a recently discovered kagome QSL candidate YCu_{3}(OH)_{6.5}Br_{2.5} up to 57 T. Above 50 T, a clear magnetization plateau at 1/3 of the saturation moment of Cu^{2+} ions is observed, supporting that this material provides an ideal platform for the kagome Heisenberg antiferromagnet. Remarkably, we found another magnetization plateau around 20 T, which is attributed to the 1/9 plateau. The temperature dependence of this plateau reveals the presence of the spin gap. The observation of 1/9 and 1/3 plateaus highlights the emergence of novel states in quantum spin systems.