Nonperturbative study of the two-frequency sine-Gordon model

The two-frequency sine-Gordon model is examined. The focus is mainly on the case when the ratio of the frequencies is 1/2, given the recent interest i n the literature. We discuss the model both in a perturbative (form factor perturbation theory) and a nonperturbative (truncated conformal space appro ach) framework, and give particular attention to a phase transition conject ured earlier by Delfino and Mussardo. We give substantial evidence that the transition is of second order and that it is in the Ising universality cla ss. Furthermore, we check the W-LR operator correspondence and conjecture t he phase diagram of the theory. (C) 2001 Elsevier Science B.V. All rights r eserved.