  A ferroelectric transition occurring simultaneously with helical spin order
has been found in both the polycrystalline and single crystal samples of
LiCuVO4. The system has Cu2+ spins (s=1/2) of the CuO2 chains formed of
edge-sharing CuO4 squares. Possibly due to the frustrated nature caused by the
competition between the nearest-neighbor and next-nearest-neighbor exchange
interactions, Cu spins exhibit, as has been already reported, the helical
magnetic order with the incommensurate modulation vector Q along the chain
direction and with the helical axis e3 perpendicular to the CuO4 squares. The
electric polarization P can be understood by the recently predicted relation (P
is proportional to Q*e3). The transition temperature has been found to be
gradually suppressed by the applied magnetic field.
