Abstract

A brief review is given of the ‘effective Lagrangian’ approach of Leutwyler, Hasenfratz and others, which describes the behaviour at low energies or temperatures, or large distances, in lattice systems which undergo a first-order transition involving spontaneous breakdown of a continuous symmetry. Universal predictions can be given, based on a continuum field theory of the massless Goldstone bosons generated by the breakdown of the symmetry, which control the behaviour in these regimes. In particular, the finite-size scaling behaviour can be predicted, in a way very similar to the predictions of conformal invariance for a two-dimensional system at a critical point. Conversely, measurements of the finite-size scaling behaviour can give estimates of the parameters of the Goldstone bosons. These points are illustrated using data for the O(2) Heisenberg spin model in (2+1) dimensions, and the XXZ antiferromagnet on a square lattice.