About the Almeida-Thouless transition line in the Sherrington-Kirkpatrick mean-field spin glass model

We consider the Sherrington-Kirkpatrick model and we prove that the quenched free energy per spin is strictly larger than the corresponding replica symmetric approximation, for all values of the temperature and of the magnetic field below the Almeida-Thouless line. This implies the breaking of replica symmetry in the whole expected spin glass phase.