Ground State of the Spin-1/2 Chain of Green Dioptase at High Fields

The gem-stone dioptase Cu6Si6O18·6H2O has a chiral crystal structure of equilateral triangular helices consisting of Cu-3d spins. It shows an antiferromagnetic order with an easy axis along c at TN = 14.5 K under zero field, and a magnetization jump at HC = 13.5 T when the field is applied along c-axis. By Si-NMR measurements, we have revealed that the high-field state is essentially the two sub-lattice structure, and that the component within ab-plane is collinear. The result indicates no apparent match with the geometrical pattern of helical spin chain.