Uncover Exotic Spin Structure in Multiferroic LiCu2O2 via Anisotropic Dielectric and Ferroelectric Responses in Magnetic Field

LiCu2O2 is the first multiferroic cuprate and its ferroelectricity is induced by complex magnetic ordering in ground state, which is still controversial today. Herein, we systematically investigated the dielectric and ferroelectric behavior of nearly untwinned LiCu2O2 single crystals in different external magnetic fields. The highly anisotropic responses in magnetic fields apparently contradict the prevalent bc- or ab- plane cycloidal spin pictures. Our observations give convincing evidence supporting another new spin spiral model in which the normal of spin helix plane is along the diagonal of CuO4 squares which form the spin chains by edge-sharing. Further analysis suggests that in ground state the spin helix is elliptical and in the intermediate state the present c-axis collinear SDW picture needs appropriate modifications. Our studies show that the di- and ferro- electric responses can be new powerful probes for the characterization of complex spin structures due to the close tie between magnetic and electric orders.