  Magnetoelectric properties were investigated for an S=1/2 chain
antiferromagnet CuCl2, which turns out to be the first example of non-chalcogen
based spiral-spin induced multiferroics. Upon the onset of helimagnetic order
propagating along the b-axis under zero magnetic field (H), we found emergence
of ferroelectric polarization along the c-axis. Application of H along the
b-axis leads to spin-flop transition coupled with drastic suppression of
ferroelectricity, and rotation of H around the b-axis induces the rotation of
spin-spiral plane and associated polarization direction. These behaviors are
explained well within the framework of the inverse Dzyaloshinskii-Moriya model,
suggesting the robustness of this magnetoelectric coupling mechanism even under
the strong quantum fluctuation.
