Ferroelectricity in anS=1/2Chain Cuprate

We report our discovery of ferroelectricity in the spiral-magnetic state in the quantum quasi-one-dimensional (1D) $S=1/2$ magnet of ${\mathrm{LiCu}}_2{\mathrm{O}}_2$. 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 $H_a$. ${\mathrm{LiCu}}_2{\mathrm{O}}_2$ 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 ${\mathrm{LiCu}}_2{\mathrm{O}}_2$.