Large quantum fluctuations in the strongly coupled spin-1/2 chains of green dioptase Cu6Si6O18•6H2O

We unravel the magnetic nature of the mineral dioptase Cu6Si6O18 center dot 6H(2)O and show that strong quantum fluctuations can be realized in an essentially framework-type spin lattice of coupled chains, thus neither frustration nor geometric low dimensionality are prerequisites. We present a microscopic magnetic model for the green dioptase. Based on full-potential density-functional theory calculations, we find two relevant couplings in this system: an antiferromagnetic coupling J(c), forming spiral chains along the hexagonal c axis, and an interchain ferromagnetic coupling J(d) within structural Cu2O6 dimers. To refine the J(c) and J(d) values and to confirm the proposed spin model, we perform quantum Monte Carlo simulations for the dioptase spin lattice. The derived magnetic susceptibility, the magnetic ground state, and the sublattice magnetization are in remarkably good agreement with the experimental data. The refined model parameters are J(c)=78 K and J(d)=-37 K with J(d)/J(c)similar or equal to-0.5. Despite the apparent three-dimensional features of the spin lattice and the lack of frustration, strong quantum fluctuations in the system are evidenced by a broad maximum in the magnetic susceptibility, a reduced value of the Neel temperature T-N similar or equal to 15 K << Jc, and a low value of the sublattice magnetization m=0.55 mu(B). All these features should be ascribed to the low coordination number of 3 that outbalances the three-dimensional nature of the spin lattice.