Evidence of a first-order smectic-hexatic transition and its proximity to a tricritical point in smectic films

Experimental and theoretical studies of a smectic-A-hexatic-B transition in freely suspended films of thickness 2-10 mu m of the n-pentyl-4'-n pentanoyloxy-biphenyl-4-carboxylate (54COOBC) compound are presented. X-ray investigations revealed a discontinuous first-order transition into the hexatic phase. The temperature region of two-phase coexistence near the phase transition point diminishes with decreasing film thickness. The width of this temperature region as a function of the film thickness was derived on the basis of a Landau mean-field theory in the vicinity of a tricritical point (TCP). Close to TCP the surface hexatic-B order penetrates anomalously deep into the film interior.