  Two kagome compounds, volborthite Cu3V2O7(OH)2-2H2O and herbertsmithite
ZnCu3(OH)6Cl2, are compared in order to derive information about the intrinsic
properties of the spin-1/2 kagome antiferromagnet. Volborthite shows a broad
maximum at T ~ J / 4 and the approach at T = 0 to a large finite value of the
bulk magnetic susceptibility chi_bulk as well as the local susceptibility
chi_local from NMR measurements. These must be intrinsic properties for the
spin-1/2 kagome antiferromagnet, as similar behavior has also been reported in
chi_local for herbertsmithite [Olariu A et al. 2008 Phys. Rev. Lett., 100
087202]. Impurity effects that may significantly influence the bulk properties
are discussed.
