The experimental quest for a quantum spin-liquid state (QSL) in frustrated magnetic systems serves fundamental scientific interests, as this intriguing quantum phase provides excellent grounds for discovering exotic collective phenomena. The S=1/2 kagome Heisenberg antiferromagnet ZnCu3(OH)6Cl2 (herbertsmithite) is the leading contender for an experimental realization of a QSL. The recent discovery of a continuum of spinon excitations using inelastic neutron scattering [1] has stimulated intense research into its physical properties. However, the nature of the paramagnetic ground state in this material remains highly debated, primarily owing to the difficulty in revealing the intrinsic magnetic behavior of the kagome lattice from defect contributions.
We used 2D and 17O NMR to probe the local behavior of spin susceptibility and spin dynamics, which provided invaluable insights into the nature of defects and their potential influence on the kagomé spin lattice. More importantly, we demonstrate that the intrinsic spin susceptibility χ_kagome tends asymptotically to zero below T~0.03J, where J ~ 200K is the Cu-Cu superexchange interaction. Combined with the magnetic field dependence of the χ_kagome observed at low temperatures, our results provide direct evidence for a QSL state with a finite gap ∆ = 0.03~ 0.07J realized in ZnCu3(OH)6Cl2 [2].

Reference:
[1] T. H. Han, J. S. Helton, S. Chu, D. G. Nocera, J. A. Rodriguez-Rivera, C. Broholm   and Y. S. Lee, Nature 492, 406 (2012).
[2] M. Fu, T. Imai, T. H. Han and Y. S. Lee, Science 350, 655 (2015).