  The ferroelectricity of the spiral magnets LiCu2O2 and LiCuVO4 was examined
by calculating the electric polarizations of their spin spiral states on the
basis of density functional theory with spin-orbit coupling. Our work
unambiguously reveals that spin-orbit coupling is responsible for the
ferroelectricity with the primary contribution from the spin-orbit coupling on
the Cu sites, but the asymmetric density distribution responsible for the
electric polarization occurs mainly around the O atoms. The electric
polarization is calculated to be much greater for the ab- than for the bc-plane
spin spiral. The observed spin-spiral plane is found to be consistent with the
observed direction of the electric polarization for LiCuVO4, but inconsistent
for LiCu2O2.
