Critical Casimir forces in the presence of random surface fields

We study critical Casimir forces (CCFs) f_C for films of thickness L which in the three-dimensional bulk belong to the Ising universality class and which are exposed to random surface fields (RSFs) on both surfaces. We consider the case in which, in the absence of RSFs, the surfaces of the film belong to the surface universality class of the so-called ordinary transition. We carry out a finite-size scaling analysis and show that for weak disorder, CCFs still exhibit scaling, acquiring a random field scaling variable w that is zero for pure systems. We confirm these analytic predictions by Monte Carlo (MC) simulations. Moreover, our MC data show that f_C varies as f_C(w →0 ) -f_C(w =0 ) ∼w² . Asymptotically, for large L , w scales as w ∼L^-0.26→0 , indicating that this type of disorder is an irrelevant perturbation of the ordinary surface universality class. However, for thin films such that w ≃1 , we find that the presence of RSFs with vanishing mean value increases significantly the strength of CCFs, as compared to systems without them, and it shifts the extremum of the scaling function of f_C toward lower temperatures. But f_C remains attractive.