Scaling of Bond-Orientational Order Parameters in A 54COOBC Two-Dimensional Film

Abstract

Electron-diffraction measurements have been conducted on thin free-standing liquid crystal 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, despite the apparent absence of herringbone order. We have determined the 6n-fold bond-orientational order (BOO) parameters C_6n and positional correlation length ζ in a two-layer film as a function of temperature in the hexatic-B phase. The results of C_6n are consistent with the scaling relation C_6n = C₆°⁽ⁿ⁾, where σ(n) = n ², as expected in a two-dimensional XY system.