Multiferroicity due to nonstoichiometry in the chain cuprate LiVCuO₄

The recently observed multiferroic behavior in the s=1/2 chain cuprate LiVCuO_4 (LiCuVO_4) with edge-shared CuO_4 plaquettes and helical spin ordering does not agree with the existing theories such as a spin-current scenario. We argue that the effect can be consistently explained, if the exchange-induced electric polarization on the out-of-chain Cu^2+ centers substituting for Li-ions in LiVCuO_4 is taken into account. These substituent centers are proved to be an effective probe of the spin incommensurability and the magnetic field effects.