A nonperturbative study of phase transitions in the multi-frequency sine-Gordon model

The phase spaces of the two- and three-frequency sine-Gordon models are examined in the framework of the truncated conformal space approach. The focus is mainly on a tricritical point in the phase space of the three-frequency model. We give substantial evidence that this point exists. We also find the critical line in the phase space and present TCSA data showing the change of the spectrum on the critical line as the tricritical endpoint is approached. We find a few points of the line of first-order transition as well.