Ring exchange in low-dimensional spin systems

Abstract

The isotropic S=12 antiferromagnetic spin ladder with additional four-spin ring exchange is studied perturbatively in the strong coupling regime with the cluster expansion technique, and by bosonization in the weak coupling limit. It is found that a sufficiently large amount of ring exchange leads to a second-order phase transition into a spontaneously dimerized phase. The shape of the phase boundary in the vicinity of the exactly known transition point is obtained. The relevance of ring exchange for materials with 2D CuO2 planes is discussed.