On the Theory of Magnetoelectric Coupling in LiCu₂O₂

A microscopic theory is proposed for coupling of copper spins with an external electric field in the magnetically ordered phase of LiCu₂O₂. The expressions are derived for the dipole moment components of exchange-coupled pairs of copper spins, as well as expressions for describing the macroscopic polarization in terms of the angles determining spatial orientations of spin rotation planes (helixes) in copper bichains. It is shown that zero electric polarization in isostructural compound NaCu₂O₂ can be explained by the difference in the spin structures in NaCu₂O₂ and LiCu₂O₂.