The McCoy-Wu Model in the Mean-field Approximation

Abstract

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 $β\approx 3.6$ and $β_1 \approx 4.1$ in the bulk and at the surface of the system, respectively. The singularity of the specific heat is characterized by an exponent $α\approx -3.1$. The samples reduced critical temperature $t_c=T_c^{av}-T_c$ has a power law distribution $P(t_c) \sim t_c^ω$ and we show that the difference between the values of the critical exponents in the pure and in the random system is just $ω\approx 3.1$. Above the critical temperature the thermodynamic quantities behave analytically, thus the system does not exhibit Griffiths singularities.

LaTeX file with iop macros, 13 pages, 7 eps figures, to appear in J. Phys. A