Critical phenomena and quenched disorder

Theoretical ideas for deriving singularities of thermodynamical functions at the second-order phase transitions in spin systems with weak quenched disorder are considered. In particular, p-component vector magnets and the two-dimensional Ising model with disorder in spin—spin interactions are studied. Generalisation of the traditional renormalisation-group scheme, which takes into account non-perturbative spin-glass degrees of freedom, is proposed. Low-temperature properties and the phase transition in the Ising systems with quenched random fields are also considered.