Gapless spin liquid behavior in a kagome Heisenberg antiferromagnet with randomly distributed hexagons of alternate bonds

We demonstrate that the single crystal of YCu3[OH(D)]6.5Br2.5 (YCOB) is a kagome Heisenberg antiferromagnet without evident orphan spins (<<0.8%). The site mixing between polar OH- and nonpolar Br- causes local distortions of Cu-O-Cu exchange paths and gives rise to 70(2)% of randomly distributed hexagons of alternate bonds (similar to J1 - AJ and J1 + AJ) and the rest of the almost-uniform hexagons (similar to J1) on the kagome lattice. Simulations of the random exchange model with AJ/J1 = 0.7(1) show good agreement with experimental observations, including the weak upturn seen in susceptibility and the slight polarization in magnetization. Despite the average antiferromagnetic coupling of J1 similar to 60 K, no conventional freezing is observed down to T similar to 0.001J1, and the raw specific heat exhibits a nearly quadratic temperature dependence below 1 K similar to 0.02J1, phenomenologically consistent with a gapless (spin gap .0.025J1) Dirac quantum spin liquid (QSL). Our result sheds light on the theoretical understanding of the randomness-relevant gapless QSL behavior in YCOB, as well as in other relevant materials.