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

Abstract

We demonstrate that the single crystal of ${\mathrm{YCu}}_{3}$[OH(D)${\mathrm{]}}_{6.5}{\mathrm{Br}}_{2.5}$ (YCOB) is a kagome Heisenberg antiferromagnet without evident orphan spins ($\ensuremath{\ll}0.8$%). The site mixing between polar ${\mathrm{OH}}^{\ensuremath{-}}$ and nonpolar ${\mathrm{Br}}^{\ensuremath{-}}$ causes local distortions of Cu--O--Cu exchange paths and gives rise to 70(2)% of randomly distributed hexagons of alternate bonds ($\ensuremath{\sim}{J}_{1}\ensuremath{-}\mathrm{\ensuremath{\Delta}}J$ and ${J}_{1}+\mathrm{\ensuremath{\Delta}}J$) and the rest of the almost-uniform hexagons ($\ensuremath{\sim}{J}_{1}$) on the kagome lattice. Simulations of the random exchange model with $\mathrm{\ensuremath{\Delta}}J/{J}_{1}$ = 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 ${J}_{1}\ensuremath{\sim}60$ K, no conventional freezing is observed down to $T\ensuremath{\sim}0.001{J}_{1}$, and the raw specific heat exhibits a nearly quadratic temperature dependence below 1 K $\ensuremath{\sim}0.02{J}_{1}$, phenomenologically consistent with a gapless (spin gap $\ensuremath{\le}0.025{J}_{1}$) 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.