Abstract
We consider a spin- 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 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.
- Received 20 June 2007
DOI:https://doi.org/10.1103/PhysRevLett.100.017202
©2008 American Physical Society