Ground-state structural transitions in small clusters within the effective temperature framework

Ground-state structural transitions in finite-sized systems are studied in the Hubbard model. We use the concept of an effective temperature γ that is directly related to the interactions among electrons. We have studied all the possible clusters, with nearest-neighbour hopping, for two, three and four sites. Furthermore, we have also analysed the octahedron and the seven-site Apollonian networks. We show that the clusters in which there is a ground-state phase transition are antiferromagnetically frustrated. The low temperature dependence for the specific heat and entropy allow us to infer that there is a V-shaped universal phase in finite clusters.