The kagome Heisenberg antiferromagnet YCu3(OH)6.5Br2.5: A review.

Quantum spin liquids (QSLs), first proposed by P. W. Anderson back in 1973 through the resonating-valence-bond state, are expected to be central to understanding high-temperature superconductivity and advancing topological quantum computation. However, conclusive experimental evidence for QSLs remains elusive, largely due to two factors: first, most two-dimensional strongly frustrated spin models are not exactly solvable, leading to inconsistent results across numerical methods; second, real materials often include spin-spin interaction perturbations that disrupt the fragile QSL ground state. This review focuses on the kagome Heisenberg antiferromagnet (KHA), which is considered a promising experimental realization of QSLs. Among the existing KHA candidates, YCu₃(OH)_6.5Br_2.5(YCOB) stands out as the most promising, showing no conventional magnetic ordering down to 50 mK despite a strong antiferromagnetic coupling of ∼ 60 K. This paper reviews key experimental and theoretical studies on YCOB, addressing ongoing challenges and future directions.