Gruppo di Fisica Statistica

In systems displaying a bulk first-order transition the order parameter may vanish continuously at a free surface, a phenomenon which is called surface-induced disorder. In the presence of surface-induced disorder the correlation lengths, parallel and perpendicular to the surface, diverge at the bulk transition point. In this way the surface induces an anisotropic power-law singular behavior for some bulk quantities. For example, in a finite system of transverse linear size L, the response functions diverge as L (d-1)z+1, where d is the dimension of the system and z is the anisotropy exponent. We present a general scaling picture for this anisotropic discontinuity fixed point. Our phenomenological results are confronted with analytical and numerical calculations on the two-dimensional q-state Potts model in the large-q limit. The scaling results are demonstrated to apply also for the same model with a layered, Fibonacci-type modulation of the couplings for which the anisotropy exponent is a continuous function of the strength of the quasiperiodic perturbation.