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Accueil du site > Publications > Publications 2008 > Spin gap and string order parameter in the ferromagnetic Spiral Staircase Heisenberg Ladder : a quantum Monte Carlo study

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 Webmaster - 19 juin 2007

Phys. Rev. Lett. 100, 017202 (2008), arxiv:0706.2769

We consider a spin-1/2 ladder with a ferromagnetic rung coupling J_\perp and inequivalent chains. This model is obtained by a twist (\theta) deformation of the ladder and interpolates between the isotropic ladder (\theta=0) and the SU(2) ferromagnetic Kondo necklace model (\theta=\pi). We show that the ground state in the (\theta,J_\perp) 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 \Delta/J_\| \propto J_\perp/J_\| for \theta < \theta_c \simeq 8\pi/9 to \Delta/J_\| \propto |J_\perp/J_\| |^2 for \theta > \theta_c. Those results are obtained on the basis of large scale Quantum Monte Carlo calculations.