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

TL;DR

This paper explains the unusual critical behavior of heat capacity near the smectic A--hexatic phase transition by modeling coupled fluctuations of orientational and positional orders, aligning theory with experimental data.

Contribution

It introduces a theoretical model that accounts for coupled order parameter fluctuations, explaining the heat capacity behavior beyond traditional universality class predictions.

Findings

Calculated specific heat temperature dependence matches experimental data.

Positional correlation length exceeds hexatic correlation length near transition.

Model rationalizes positive critical exponent observed experimentally.

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.