Neutron diffraction and bulk measurements are used to determine the nature of the low-temperature ordered state in ${\mathrm{LiCu}}_2{\mathrm{O}}_2$, a $S=1/2$ spin-chain compound with competing interactions. The spin structure is found to be helimagnetic, with a propagation vector $(0.5,\zeta ,0)$, $\zeta =0.174$. The nearest-neighbor exchange constant and frustration ratio are estimated to be $J_1=5.8\mathrm{meV}$ and $J_2/J_1=0.29$, respectively. For idealized quantum spin chains, these parameter values would signify a gapped spin-liquid ground state with commensurate spin correlations. The observed temperature dependence of the magnetic propagation vector in ${\mathrm{LiCu}}_2{\mathrm{O}}_2$ is attributed to a competition between incommensurate helimagnetism in the classical spin model and commensurability in the quantum case. It is also proposed that long-range ordering in ${\mathrm{LiCu}}_2{\mathrm{O}}_2$ is facilitated by intrinsic nonstoichiometry.

• Received 6 October 2003

DOI:https://doi.org/10.1103/PhysRevLett.92.177201