Abstract
We have obtained exact results for the Ising model on a hierarchical lattice incorporating three key features characterizing many real-world networks—a scale-free degree distribution, a high clustering coefficient, and the small-world effect. By varying the probability of long-range bonds, the entire spectrum from an unclustered, non-small-world network to a highly clustered, small-world system is studied. Using the self-similar structure of the network, we obtain analytic expressions for the degree distribution and clustering coefficient for all , as well as the average path length for and . The ferromagnetic Ising model on this network is studied through an exact renormalization-group transformation of the quenched bond probability distribution, using up to renormalized probability bins to represent the distribution. For , we find power-law critical behavior of the magnetization and susceptibility, with critical exponents continuously varying with , and exponential decay of correlations away from . For , in fact where the network exhibits small-world character, the critical behavior radically changes: We find a highly unusual phase transition, namely an inverted Berezinskii-Kosterlitz-Thouless singularity, between a low-temperature phase with nonzero magnetization and finite correlation length and a high-temperature phase with zero magnetization and infinite correlation length, with power-law decay of correlations throughout the phase. Approaching from below, the magnetization and the susceptibility, respectively, exhibit the singularities of and , with and positive constants. With long-range bond strengths decaying with distance, we see a phase transition with power-law critical singularities for all , and evaluate an unusually narrow critical region and important corrections to power-law behavior that depend on the exponent characterizing the decay of long-range interactions.
12 More- Received 12 December 2005
DOI:https://doi.org/10.1103/PhysRevE.73.066126
©2006 American Physical Society