  The two-dimensional (2D) 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 distinct spin gap, whose magnitude estimated by the
plateau width is approximately 10% of the exchange interaction. The observation
of 1/9 and 1/3 plateaus highlights the emergence of novel states in quantum
spin systems.

Strongly Correlated Electrons