Small-world phenomena in physics: the Ising model

The Ising system with a small fraction of random long-range interactions is the simplest example of small-world phenomena in physics. Considering the latter both in an annealed and in a quenched state we conclude that: (a) the existence of random long-range interactions leads to a phase transition in the one-dimensional case and (b) there is a minimal average number p of these interactions per site ( p <1 in the annealed state, and p ≃1 in the quenched state) needed for the appearance of the phase transition. Note that the average number of these bonds, pN /2, is much smaller than the total number of bonds, N 2 /2.