Coexistence of dimerization and long-range magnetic order in the frustrated spin-chain system ${\mathrm{LiCu}}_2{\mathrm{O}}_2:$ Inelastic light scattering study

Raman scattering studies of the frustrated spin chain system ${\mathrm{LiCu}}_2{\mathrm{O}}_2$ are reported. Two transitions into a magnetically ordered phase (taken place at temperatures $\sim 9$ and $\sim 24\mathrm{K})$ have been confirmed from the analysis of optical properties of the samples. Interestingly, two different magnetic excitations, seen at 100 and $110{\mathrm{cm}}^{-1}$ in the magnetically ordered phase superimpose each other independently, indicating a coherent coexistence of long-range magnetic order and dimerization. The observed phenomenon is attributed to magnetostructural peculiarities of ${\mathrm{LiCu}}_2{\mathrm{O}}_2$ leading to the intrinsic presence of magnetic/nonmagnetic impurities on a nanometer scale. In particular, magnetic impurities play a significant role in driving the transition from an incommensurate state to a Néel ordered one at 9 K.