Groupe de Physique Statistique

We consider a system with randomly layered ferromagnetic bonds (McCoy-Wu model) and study its critical properties in the frame of mean-field theory. In the low-temperature phase there is an average spontaneous magnetization in the system, which vanishes as a power law at the critical point with the critical exponents β âo/oo^ 3.6 and β1 âo/oo^ 4.1 in the bulk and at the surface of the system, respectively. The singularity of the specific heat is characterized by an exponent α âo/oo^ -3.1. The samples reduced critical temperature tc = Tavc - Tc has a power law distribution P(tc) â^Œ tÏo/ooc and we show that the difference between the values of the critical exponents in the pure and in the random system is just Ïo/oo âo/oo^ 3.1. Above the critical temperature the thermodynamic quantities behave analytically, thus the system does not exhibit Griffiths singularities.