Why the smectic A -- hexatic phase transition does not follow its universality class?

Abstract: We resolve the old riddle related to the critical behavior of the heat capacity near the smectic A -- hexatic second order phase transition. Experiment suggests a "large" positive specific heat critical exponent inconsistent with the universality class for this phase transition implying the very small negative exponent. We show that essential features of the heat capacity for the smectic A -- hexatic phase transition can be rationalized in the framework of a theoretical model treating jointly fluctuations of the hexatic orientational order and of the positional (translational) order parameters. Assuming that the positional (translational) correlation length is larger than the hexatic correlation length, we calculate a temperature dependence of the specific heat in the critical region near the smectic A -- hexatic phase transition. Our results are in a quantitative agreement with the calorimetric experimental data.