Thermodynamics of Fateev's models in the presence of external fields

We study the Thermodynamic Bethe Ansatz equations for a one-parameter quant um field theory recently introduced by V.A. Fateev. The presence of chemica l potentials produces a kink condensate that modifies the excitation spectr um. For some combinations of the chemical potentials an additional gapless mode appears. Various energy scales emerge in the problem. An effective fie ld theory, describing the low energy excitations, is also introduced. (C) 2 000 Elsevier Science B.V. All rights reserved.