Ferroelectricity in an S=1/2 chain cuprate

We report our discovery of ferroelectricity in the spiral-magnetic state in the quantum quasi-one-dimensional (1D) S = 1/2 magnet of LiCu2O2. Electric polarization (P) emerges along the c direction below the spiral-magnetic order temperature, but changes from the c to a axis when magnetic fields (H) are applied along the b direction. We also found that P-c increases with H-c, and P-a appears with H-a. LiCu2O2 in zero field appears to be the first ferroelectric cuprate and also a prototypical example of the 1D spiral-magnetic ferroelectrics. However, the unexpected behavior in H may demonstrate the complexity of the ordered spin configuration, inherent in the 1D S = 1/2 magnet of LiCu2O2.