Magnetic ground state and magnetic excitations in black dioptase Cu₆Si₆O₁₈

The low-temperature magnetic properties and magnetic structure of fully dehydrated black dioptase Cu₆Si₆O₁₈ have been studied by single-crystal neutron diffraction and magnetization measurements in magnetic fields up to 30 T. The intrachain $J_c$ and interchain $J_{ab}$ interactions as well as the anisotropy of the exchange coupling $J_c$ have been determined using inelastic neutron-scattering techniques. Zero-field antiferromagnetic order at $T_{\text{N}}=6.7\text{K}$ can be described by a commensurate propagation vector of $k=(0,0,3/2)$ with respect to the hexagonal $R\overline{3}:\text{H}$ unit cell. The Cu magnetic moments are aligned antiferromagnetically along the c axis with about 12° tilt and are coupled ferromagnetically between the chains. The high-field magnetization measurements provide strong evidence for the presence of a spin-flop phase above 8 T. We found that $J_c$ in black dioptase is significantly increased, while $J_{ab}$ is much weaker compared to its counterpart, green dioptase Cu₆[Si₆O₁₈] 6H₂O. We suggest that black dioptase behaves like a nearly ideal $S=1/2$ antiferromagnetic Heisenberg spiral chain with enhanced quantum fluctuations and weak spinon confinement $J_{ab}/J_c\sim 0.02$ .