Scaling of 6n-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 alms 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 = C-6(sigma(n)), where sigma(n) = n(2), as expected in a two-dimensional XY system.