LIGHT-SCATTERING STUDY OF MELTING IN SMECTIC LIQUID-CRYSTAL FILMS

We use quasielastic light scattering to investigate bond-orientational order in freely suspended, tilted smectic films of the liquid-crystal 4-(2-methylbutyl)-phenyl-4'-(octyloxy)-(l,l')-biphenyl-4-carboxylate (8OSI). We find that the scattering for bend and splay in-plane fluctuations of the magnetically aligned smectic-C director in thin (2-4-layer) and thick (almost-equal-to 1000-layer) samples is well described by a simple elastic free energy coupling the bond-orientational to the molecular-tilt order parameter, combined with the form factor for a tilted smectic film. We discuss results for the elasticity and overdamped dynamics of the coupled fluctuations in light of the theory of defect-mediated melting in two dimensions. In addition, we draw an interesting comparison with results obtained on the closely related compound 4-(2'-methylbutyl)-phenyl-4-octylbiphenyl-4-carboxylate 8SI*), which suggests that the magnitude of the intrinsic bond-tilt coupling is surprisingly different in the two materials and that the melting of the two-dimensional (2D) solid phase may involve different dislocation unbinding processes. Finally, orientational scans of the in-plane wave vector in the lower hexatic phase of 2D 8OSI films reveal evidence of pronounced temperature-dependent changes in the liquid-crystal director alignment, which we account for with a simple phenomenological model for the effect on the light scattering of textural defects in the 2D director field.