NONLINEAR EXCITATIONS IN THE CRITICAL REGION

The free energy transformation due to fluctuations is investigated in an exactly solvable model, This model accounts for the fluctuation int eraction in a reduced manner and leads to a realistic estimation for t he free energy. In particular it gives a nice critical exponent delta = 5. It is shown that in spite of the monotonic character of the effec tive free energy in the critical region the properties of the system s hould be described on the basis of the phi6 model. Localized nonlinear excitations are found to be possible with a profile rather like that known as a ''bump'' near the point of the first-order phase transition .