Landau theory for smectic-A–hexatic-B coexistence in smectic films

Physical Review E - Statistical, Nonlinear, and Soft Matter Physics. 2019. Vol. 100. Article 022705.

Kats E. I., Lebedev V., Pikina E.

We explain theoretical peculiarities of the smectic-A–hexatic-B equilibrium phase coexistence in a finitetemperature
range recently observed experimentally in free-standing smectic films [I. A. Zaluzhnyy et al.,
Phys. Rev. E 98, 052703 (2018)]. We quantitatively describe this unexpected phenomenon within Landau phase
transitions theory assuming that the film state is close to a tricritical point.We found that the surface hexatic order
diminishes the phase coexistence range as the film thickness decreases, shrinking it to zero at some minimal film
thickness Lc, of the order of a few hexatic correlation length. We established universal laws for the temperature
width of the phase coexistence range in terms of the reduced variables. Our theory is in agreement with the
existing experimental data.

Keywords: phase coexistence, smectic films, tricritical point, surface hexatic order