Correlation between Spin Helicity and an Electric Polarization Vector in Quantum-Spin Chain MagnetLiCu2O2

Measurements of polarized neutron scattering were performed on a $S=1/2$ chain multiferroic ${\mathrm{LiCu}}_2{\mathrm{O}}_2$. In the ferroelectric ground state with the spontaneous polarization along the $c$ axis, the existence of transverse spiral spin component in the $bc$ plane was confirmed. When the direction of electric polarization is reversed, the vector spin chirality as defined by ${\mathbf{C}}_{ij}={\mathbf{S}}_i\times {\mathbf{S}}_j$ ($i$ and $j$ being the neighboring spin sites) is observed to be reversed, indicating that the spin-current model or the inverse Dzyaloshinskii-Moriya mechanism is applicable even to this $e_g$-electron quantum-spin system. Differential scattering intensity of polarized neutrons shows a large discrepancy from that expected for the classical-spin $bc$-cycloidal structure, implying the effect of large quantum fluctuation.