Fluctuation-induced interaction in smectic films under an ordering external field

Within a quadratic functional integral approach, we investigate the influence of an external magnetic field in the fluctuation-induced surface–surface interaction of free-standing smectic liquid crystals. We find that the typical 1/l decay of the Casimir-like contribution to the free-energy of a film with thickness l in the absence of external fields is replaced by a faster 1/l2 decay for finite fields. We show the existence of a characteristic thickness delimiting the crossover between these distinct regimes which continuously decreases as stronger fields are considered.