In this paper, we report inelastic neutron scattering measurements of the magnetic excitations of green dioptase ${\mathrm{Cu}}_6\left[{\mathrm{Si}}_6{\mathrm{O}}_{18}\right]·6{\mathrm{H}}_2\mathrm{O}$. 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-$\frac{1}{2}$ Hamiltonian that includes antiferromagnetic nearest-neighbor intrachain coupling $J_c=10.6\left(1\right)$ meV, ferromagnetic interchain coupling $J_{ab}=-1.2\left(1\right)$ meV, and exchange anisotropy $\mathrm{\Delta }{J}_c=0.14\left(1\right)$ meV. We calculated the sublattice magnetization to be strongly reduced, $\sim 0.39{\mu }_{\mathrm{B}}$. This appears compatible with a reduced Néel temperature, $T_{\mathrm{N}}=14.5\text{K}\ll J_c$, and can be explained by a presence of quantum spin fluctuations.

• Received 24 September 2015

DOI:https://doi.org/10.1103/PhysRevB.93.064426