We analyze the effects of a trimerized modulation in a quantum spin S=21 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.
@article{arxiv.cond-mat/0612271,
title = {Quantum phase transitions in trimerized zig-zag spin ladders},
author = {H. D. Rosales and D. C. Cabra and M. D. Grynberg and G. L. Rossini and T. Vekua},
journal= {arXiv preprint arXiv:cond-mat/0612271},
year = {2007}
}
Comments
Numerical analysis improved, 4 figures added. Final version to appear in Phys. Rev. B