Inverse Mermin-Wagner theorem for classical spin models on graphs

In this paper we present the inversion of the Mermin-Wagner theorem on grap hs, by proving the existence of spontaneous magnetization at finite tempera ture for classical spin models on transient on the overage graphs, i.e., gr aphs where a random walker returns to its starting point with an average pr obability (F) over bar<1. This result, which is here proven for models with O(n) symmetry, includes as a particular case n=1, providing a very general condition for spontaneous symmetry breaking on inhomogeneous structures ev en for the Ising model. [S1063-651X(99)12208-6].