Nature of the smectic-[Formula Presented]—hexatic-[Formula Presented]—crystal-[Formula Presented] transitions of a liquid-crystal compound

High-resolution heat-capacity, optical reflectivity, and electron-diffraction measurements have been conducted on thin free-standing films of a liquid-crystal compound exhibiting the smectic-[Formula Presented]—hexatic-[Formula Presented]—crystal-[Formula Presented] transitions. Despite the absence of herringbone order, the liquid-hexatic transition in extremely thin films has been found to show a pronounced pretransitional heat-capacity anomaly, in disagreement with the predictions of two-dimensional melting theory. In addition, both the surface-enhanced layer-by-layer transition near the smectic-[Formula Presented]—hexatic-[Formula Presented] and an unexpected layer thinning transition above the bulk isotropic-smectic-[Formula Presented] transition temperature have been observed.