Gapless spin liquid behavior in a kagome Heisenberg antiferromagnet with randomly distributed hexagons of alternate bonds
Jiabin Liu, Yuan Long, Xuan Li, Boqiang Li, Kan Zhao, Haijun Liao, Yuesheng Li
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.