Comparison of spin dynamics in green and black dioptase: from classical chains to quantum fluctuations*

Magnetic excitations of a gem crystal green dioptase Cu₆[Si₆O₁₈]^.6H₂O and fully dehydrated black dioptase Cu₆Si₆O₁₈ have been studied by inelastic neutron scattering. The intrachain J_c and interchain J_ab interactions as well as the anisotropy of the J_c have been determined. The observed spectrum of green dioptase contains two sharp 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. We found that dehydration has a dual effect on the magnetic interactions: it significantly increases the intrachain exchange interaction J_c, and simultaneously decouples the chains by suppressing J_ab. The spin dynamics of green dioptase are dominated by conventional magnon excitations, whereas the continuum of excitations observed in black dioptase indicates the presence of fractionalized spinons. We suggest that black dioptase behaves like a nearly ideal spin-1/2 antiferromagnetic Heisenberg spiral chain with enhanced quantum fluctuations and weak spinon confinement J_ab/J_c~0.02.