Evidence for a Gapped Spin-Liquid Ground State in a Kagome Heisenberg Antiferromagnet

摘要

The kagome Heisenberg antiferromagnet is a leading candidate in the searchfor a spin system with a quantum spin-liquid ground state. The nature of itsground state remains a matter of great debate. We conducted 17-O single crystalNMR measurements of the S=1/2 kagome lattice in herbertsmithiteZnCu$_3$(OH)$_6$Cl$_2$, which is known to exhibit a spinon continuum in thespin excitation spectrum. We demonstrate that the intrinsic local spinsusceptibility $\chi_{kagome}$ deduced from the 17-O NMR frequency shiftasymptotes to zero below temperature T ~ 0.03 J, where J ~ 200 K is the Cu-Cusuper-exchange interaction. Combined with the magnetic field dependence of$\chi_{kagome}$ we observed at low temperatures, these results imply that thekagome Heisenberg antiferromagnet has a spin-liquid ground state with a finitegap.