Antisite disorder and the magnetic groundstate of an experimental spin 1/2 kagome antiferromagnet

Physical realisations of the spin 1/2 kagome antiferromagnet have been long sought-after because it is expected that the magnetic groundstate of this system can retain the full symmetry of the magnetic Hamiltonian. I will discuss recent experimental results on Zn_xCu(4-x)(OH)6Cl2 with 0 < x <1. Muon-spin relaxation, neutron diffraction and specific-heat measurements were carried out for a range of stoichiometries up to the x = 1 phase, which has been claimed to be an ideal model of the geometrically frustrated spin 1/2 kagome antiferromagnet. The interpretation of the data is complicated by the presence of antisite disorder between the Cu and the Zn sites, but a detailed analysis of the heat capacity data for compositions 0.5 < x < 1 has allowed for this to be taken into account in the further analysis of the experimental data. I will argue that the effective magnetic Hamiltonian in this narrow-band charge-transfer compound must be of lower symmetry than the Heisenberg Hamiltonian.