Phase transitions and elementary excitations in a zigzag-like spin system

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Abstract
Phase transitions in the quantum frustrated antiferromagnetic two-chain system with additional three-spin interactions of special form are studied. With the help of an exact Bethe ansatz solution it is shown that such a zigzag-like system has two critical-magnetic-field values in the ground state: One of these is the usual phase transition into a ferromagnetic state, while the second one is connected with magnetic structure doubling. Two phase transitions are revealed in the “factorization” of the dispersion law of an elementary excitation. The absence of a gap behavior in recent experiments is explained. Two types of chiral spin ordering for frustrated low-dimensional magnets are proposed.