Abstract
In this paper, we report inelastic neutron scattering measurements of the magnetic excitations of green dioptase Cu-6[Si6O18]center dot 6H(2)O. The observed spectrum contains two magnetic modes and a prominent spin gap that is consistent with the ordered ground state of Cu moments coupled antiferromagnetically in spiral chains along the c axis and ferromagnetically in ab planes on the hexagonal cell. The data are in excellent agreement with a spin-1/2 Hamiltonian that includes antiferromagnetic nearest-neighbor intrachain coupling J(c) = 10.6(1) meV, ferromagnetic interchain coupling J(ab) = -1.2(1) meV, and exchange anisotropy Delta J(c) = 0.14(1) meV. We calculated the sublattice magnetization to be strongly reduced, -0.39 mu B. This appears compatible with a reduced Neel temperature, T-N = 14.5 K << J(c), and can be explained by a presence of quantum spin fluctuations.
