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

Abstract

Experimental and theoretical studies of a smectic-$A$--hexatic-$B$ transition in freely suspended films of thickness $2--10\phantom{\rule{3.33333pt}{0ex}}\ensuremath{\mu}\mathrm{m}$ of the $n$-pentyl-${4}^{\ensuremath{'}}\ensuremath{-}n$-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.