A SEMICLASSICAL ANALYSIS OF ORDER FROM DISORDER

In this paper we present several examples of order due to disorder phe nomena and mainly focus on the Heisenberg antiferromagnet with nearest , neighbour interactions on the Husimi cactus, a system which locally has the same topology as the Kagome lattice. This system has a huge cl assical degeneracy corresponding to an extensive number of degrees of freedom. We show that unlike thermal fluctuations, quantum fluctuation s partially lift this degeneracy and favour a discrete subset of class ical ground states. In order to clarify the origin of these effects, w e have set up a general semiclassical analysis of the order from disor der phenomenon and clearly identified the differences between classica l and quantum fluctuations, This semiclassical approach also enables u s to classify various situations where a selection mechanism still occ urs. Moreover, once a discrete set of ground states has been preselect ed, our analysis suggests that tunelling processes within this set sho uld be the dominant effect underlying the strange low-energy spectrum of Kagome-like lattices.