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

TL;DR

This study computes thermodynamic properties of an anisotropic Heisenberg antiferromagnet on the kagome lattice, compares results with Volborthite experiments, and explores magnetic behavior including magnetization plateaus.

Contribution

It provides a detailed comparison between theoretical models and experimental data, highlighting the near isotropic nature of Volborthite and analyzing the effects of anisotropy on magnetic properties.

Findings

Volborthite is best modeled with nearly isotropic exchanges.

The low-energy density of states in Volborthite may be lower than in the ideal Heisenberg model.

The 1/3 magnetization plateau persists under anisotropy and extends to lower fields.

Abstract

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 kagom\'{e} 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.