Using the Ginzburg criterion, we demonstrate that the width of the critical region associated with the smectic-A–smectic-C transition is dependent on the observable studied. This affords an explanation of the results obtained for terephthal-bis-p-p’-butylaniline, which shows a specific-heat behavior consistent with a Landau-type model, unlike that of the elastic constants. This analysis could apply to other phase transitions.

• Received 29 March 1989

DOI:https://doi.org/10.1103/PhysRevLett.63.774