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Spin gap and string order parameter in the ferromagnetic Spiral Staircase Heisenberg Ladder : a quantum Monte Carlo study
C. Brünger, F. F. Assaad, S. Capponi, F. Alet, D. N. Aristov and M. N. Kiselev
par - 19 juin 2007
Phys. Rev. Lett. 100, 017202 (2008), arxiv:0706.2769
We consider a spin-1/2 ladder with a ferromagnetic rung coupling 
and inequivalent chains. This model is obtained by a twist (
) deformation of the ladder and interpolates between the isotropic ladder (
) and the SU(2) ferromagnetic Kondo necklace model (
). We show that the ground state in the (
) plane has a finite string order parameter
characterizing the Haldane phase. Twisting the chain introduces a new energy scale, which we interpret in terms of a Suhl-Nakamura interaction. As a consequence we observe a crossover in the scaling of the spin gap at weak coupling from 
for 
to 
for 
. Those results are obtained on the basis of large scale Quantum Monte Carlo calculations.