Critical Phenomena at the Surface of Systems Undergoing a Bulk First Order Transition: Are They Understood?

Systems that exhibit a first-order phase transition in the bulk, such as binary alloys where the order parameter vanishes discontinuously at some critical value of a control parameter, may show a continuous vanishing of the local order parameter at the surface. This “surface-induced disordering” is described theoretically as a variant of critical wetting, where an interface between the locally disordered surface and the ordered bulk gradually moves towards the bulk. We test this description by Monte Carlo simulations for a body centered cubic model alloy, with interactions between nearest and next nearest neighbors, for which the phase diagram in the bulk has been calculated very accurately. A critical vanishing of the order parameter components is found both for the (110) and the (100) surface. In contrast to the theory, the simulations indicate different critical exponents v from the order parameter of the B2 phase and the order parameter of the DO3 phase. Observations from simulations for the face centered cubic lattice and from experiments are also briefly discussed.