Field-induced chiral phase in isotropic frustrated spin chains

It is argued that an external magnetic field applied to an isotropic zigzag spin chain with an arbitrary spin S and antiferromagnetic nearest-neighbor and next-nearest-neighbor exchange couplings J(1) and J(2) induces a phase with spontaneously broken parity, characterized by long-range ordering of vector chirality. To show that, we use a bosonization approach for S=1/2 and S=1, valid in the limit of a weak zigzag interaction J(1)/J(2)<< 1, as well as an effective large-S theory applicable in the vicinity of the saturation field. Relevance to real materials and the possibility of experimental observation of the chiral phase are discussed.