Finite Size Effects in Thin Film Simulations
Phase transitions in thin films are discussed, with an emphasis on Ising-type systems (liquid-gas transition in slit-like pores, unmixing transition in thin films, orderdisorder transitions on thin magnetic films, etc.) The typical simulation geometry then is a L xL x D system, where at the low confining L x L surfaces appropriate boundary “fields” are applied, while in the lateral directions periodic boundary conditions are used. In the z-direction normal to the film, the order parameter always is inhomogeneous, due to the boundary “fields” at the confining surfaces. When one varies the temperature T from the region of the bulk disordered phase to a temperature below the critical temperature of the bulk (T cb ), one may encounter the onset of a stratified structure, i.e. domains of appropriate sign of the order parameter form, separated by a domain wall parallel to the confining surfaces. If the boundary fields favor the same phase, a structure with two parallel interfaces may form, if the boundary fields favor opposite phases, formation of a single interface results. In both cases, this vertical phase separation is a rounded transition due to the finite size of the linear dimension D, not a sharp phase transition as in the bulk. However, for L → ∞ phase transitions at temperatures T C (D) can occur, which can be interpreted as a symmetry breaking of a quasi-two dimensional character, leading to phase separation in lateral directions if one keeps the value of the total order parameter in the thin film fixed. Phenomenological theories of these phenomena (“capillary condensation”, “interface localization transition”) will be discussed, and it will be shown that there can exist a very large correlation length in parallel direction, leading to strong finite size effects if L is not extremely large. The general considerations will be exemplified with selected Monte Carlo results.
KeywordsCoexistence Curve Thin Magnetic Film Dimensional Crossover Lateral Phase Separation Ising Universality Class
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