Cluster percolation and pseudocritical behaviour in spin models

The critical behaviour of many spin models can be equivalently formulated a s percolation of specific site-bond clusters. In the presence of an externa l magnetic field, such clusters remain well-defined and lead to a percolati on transition, even though the system no longer shows thermal critical beha viour. We investigate the 2-dimensional Ising model and the 3-dimensional O (2) model by means of Monte Carlo simulations. We find for small fields tha t the line of percolation critical points has the same functional form as t he line of thermal pseudocritical points. (C) 2001 Published by Elsevier Sc ience B.V.