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Quantum phase transitions in trimerized zig-zag spin ladders

H. D. Rosales, D. C. Cabra, M. D. Grynberg, G. L. Rossini, T. Vekua

cond-mat.str-elarXiv:cond-mat/0612271

Abstract

We analyze the effects of a trimerized modulation in a quantum spin S=12 zig-zag ladder at the magnetization plateau M=1/3. Such periodicity is argued to be stemmed from lattice deformations by phonons. The interplay between frustration and exchange modulation is well described by an effective triple sine-Gordon field theory close to the homogeneous ladder and by block-spin perturbation theory in the weakly coupled trimers regime. The characteristic triple degeneracy of the ground state for homogeneous ladders gives place to modulation driven quantum phase transitions, leading to a rich phase diagram including up-up-down, quantum plateau and gapless plateau states.

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