Scaling of 6 n-Fold Bond-Orientational Order Parameters in a Hexatic Liquid-Crystal Thin Film

Electron-diffraction measurements have been conducted on two-layer free-standing films of n-pentyl- 4^′- n-pentanoyloxy-biphenyl-4-carboxylate (54COOBC). This compound undergoes the smectic- A–hexatic- B–crystal- B phase sequence, but its thin films exhibit power-law thermal anomalies at the smectic- A–hexatic- B transition which are not predicted in the theory of defect-mediated melting in two dimensions. The films just below the transition show hexaticlike positional correlations but no equilibrium bond-orientational order. The 6n-fold bond-orientational order parameters C_6n, which can be determined at lower temperatures, are consistent with the scaling relation C_6n = C₆^σ(n), where σ(n) = n², as expected in a two-dimensional XY system.