Thermodynamic properties of the Spin-1/2 Heisenberg Antiferromagnet with Anisotropic Exchange on the Kagome Lattice: Comparison with Volborthite

Thermodynamic properties such as magnetic susceptibility and specific heat have been computed for the Heisenberg Antiferromagnet with spatially anisotropic exchange on the kagome lattice on clusters up to N=24 spins from the full spectra obtained by exact diagonalization. This approach is shown to provide a good represention of these thermodynamic properties above temperatures of about $J_{\rm av}/5$ where $J_{\rm av}$ is an average of the coupling constants. Comparison with experimental Volborthite data obtained by Hiroi {\it {et al}} [J. Phys. Soc. Jpn. {\bf 70},3377 (2001)] shows that Volborthite is best described by a model with nearly isotropic exchanges in spite of the significant distortion of the kagomé lattice of magnetic sites in this compound and suggests that additional interactions are present. Comparison of the specific heat at low temperature raise the possibility that the density of states at low energy in Volborthite might be much lower than in the Heisenberg model. Magnetization curves under an applied field of the model are also investigated. The M=1/3 plateau is found to subsist in the anisotropic case and extend to lower field with increased anisotropy. For sufficient anisotropy, this plateau would then be observable for a field reasonably accessible to experiment. The absence of a plateau well below $\sim$ 70 Teslas would further support a nearly isotropic model.