MARK KAC SEMINAR

We discuss several two-dimensional lattice models displaying a form of self-organized criticality. The behavior of these models is related to the phase transition of independent site percolation, which is now very well understood in two dimensions. In particular, we study the frozen percolation model (where connected components of vertices stop growing when they get too large), and we present a connection with forest-fire processes (where lightning hits independently each vertex with a small rate, and burns its entire connected component immediately).

This talk is based on joint works with Rob van den Berg (CWI and VU Amsterdam) and Demeter Kiss.

It is generally believed that, in the thermodynamic limit, the microcanonical description as a function of energy coincides with the canonical description as a function of temperature. However, various examples of systems for which the microcanonical and canonical ensembles are not equivalent have been identified. A complete theory of this intriguing phenomenon is still missing. Here we show that ensemble nonequivalence can manifest itself also in random graphs with topological constraints. We find that, while graphs with a given number of links are ensemble-equivalent, graphs with a given degree sequence are not. This result holds irrespective of whether the energy is nonadditive (as in unipartite graphs) or additive (as in bipartite graphs). In contrast with previous expectations, our results show that: (1) physically, nonequivalence can be induced by an extensive number of local constraints, and not necessarily by long-range interactions or nonadditivity; (2) mathematically, nonquivalence is determined by a different large-deviation behaviour of microcanonical and canonical probabilities for a single microstate, and not necessarily for almost all microstates. The latter criterion, which is entirely local, is not restricted to networks and holds in general. We also show that the relative canonical fluctuations of the constraints do not vanish, and generalise our main results to the case of multi-layer graphs with an arbitrary distribution of intra-layer and inter-layer degrees.