Coupled Antiferromagnetic spin-1/2 Chains in Green Dioptase Cu₆[Si₆O₁₈] ⋅ 6H₂O

In this paper, we report inelastic neutron scattering measurements of the magnetic excitations of green dioptase Cu₆ [Si₆ O₁₈] ∙ 6H₂O.  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 -½ Hamiltonian that includes antiferromagnetic nearest-neighbor intrachain coupling Jc = 10.6 (1) meV, ferromagnetic interchain coupling J_ab = -1.2 (1) meV, and exchange anisotropy ΔJ_c = 0.14 (1) meV.  We calculated the sublattice magnetization to be strongly reduced, ~ 0.39µ_B.  This appears compatible with a reduced Néel temperature, T_N = 14.5 K ≪ J_c, and can be explained by a presence of quantum spin fluctuations.