  LiCu2O2 is the first multiferroic cuprate to be reported and its
ferroelectricity is induced by complex magnetic ordering in ground state, which
is still in controversy today. Herein, we have grown nearly untwinned LiCu2O2
single crystals of high quality and systematically investigated their
dielectric and ferroelectric behaviours in external magnetic fields. The highly
anisotropic response observed in different magnetic fields apparently
contradicts the prevalent bc- or ab- plane cycloidal spin model. Our
observations give strong evidence supporting a new helimagnetic picture in
which the normal of the spin helix plane is along the diagonal of CuO4 squares
which form the quasi-1D spin chains by edge-sharing. Further analysis suggests
that the spin helix in the ground state is elliptical and in the intermediate
state the present c-axis collinear SDW model is applicable with some
appropriate modifications. In addition, our studies show that the dielectric
and ferroelectric measurements could be used as probes for the characterization
of the complex spin structures in multiferroic materials due to the close tie
between their magnetic and electric orderings.
