Does herbertsmithite have an intrinsic spin gap?
In plain words
Herbertsmithite, ZnCu3(OH)6Cl2, is the best-studied kagome spin-liquid material, but some copper and zinc atoms swap places. Nuclear magnetic resonance experiments disagree with each other on whether there is a small energy gap, and the defects make the data hard to interpret.
Precise statement
In ZnCu3(OH)6Cl2 ($J/k_B$ about 190 K), determine whether the kagome-plane spin excitations have a gap $\Delta > 0$ as $T \to 0$ once the contribution of Cu on interlayer Zn sites (about 5 to 15 percent) is subtracted. An answer is $\Delta/J$ with uncertainty, or an upper bound consistent with zero, reconciled across NMR, neutron and thermodynamic data.
What would settle it
Site-selective NMR and polarized neutron spectroscopy on crystals with independently measured defect content, analyzed with a model including the interlayer Cu spins.
Status in the literature
17O NMR by Fu et al. (Science 2015) reported a gap of a few percent of J, while later 17O NMR by Khuntia et al. (Nature Physics 2020) and neutron data indicate a gapless state; the role of interlayer Cu defects remains debated.