Nature of the smectic-A2-smectic-C2 transition: A calorimetric study.

Abstract

The first high-resolution specific-heat measurement on octylphenyl-2-chloro-4-(p-cyano benzoyloxy) benzoate (${\mathrm{DB}}_{8}$ClCN) is reported. At 390.2 K, the material undergoes a second-order phase transition from a bilayer smectic-A phase (Sm-${A}_{2}$) to a bilayer smectic-C phase (Sm-${C}_{2}$). On cooling, the specific heat of ${\mathrm{DB}}_{8}$ClCN is independent of temperature in the Sm-${A}_{2}$ phase and increases at ${T}_{c}$ in a steplike fashion. In the Sm-${C}_{2}$ phase, the specific heat again becomes temperature independent. This is in striking contrast to previously reported Sm-A--Sm-C transition data, all of which conform to a Landau model with an unusually large sixth-order term.