Helimagnetism in a disordered and geometrically frustrated quantum spin chain

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Neutron diffraction and bulk measurements are used to determine the nature of the low-temperature ordered state in LiCu₂O₂, a S=1/2 spin-chain compound with competing interactions. The magnetic structure is found to be helimagnetic, with a propagation vector (0.5,ζ,0), ζ=0.174. The nearest-neighbor exchange constant and frustration ratio are estimated to be J=5.8 meV and J₂/J₁=0.29, respectively. For idealized spin chains these parameter values would signify a gapped spin-liquid ground state. Long-range ordering is attributed to intrinsic non-stoichiometry.