Destruction of a first-order smectic-A→smectic-C*phase transition by dimensional crossover in free-standing films

Abstract

Shifting and destruction of phase transitions due to the finite thickness of free-standing smectic films are examined. The behavior of smectic films is studied as a function of the temperature and the thickness in a material showing a first-order smectic-A (Sm-A)→smectic-${\mathit{C}}^{\mathrm{*}}$ (Sm-${\mathit{C}}^{\mathrm{*}}$) bulk phase transition. Measurements of the optical reflectivity and direct observations in an inverted polarizing microscope reveal that the first-order phase transition between Sm-A- and Sm-${\mathit{C}}^{\mathrm{*}}$-like structures takes place in films that are thick enough (N>${\mathit{N}}_{\mathit{c}}$=90). In thin films (N<${\mathit{N}}_{\mathit{c}}$) this transition disappears. For N≊${\mathit{N}}_{\mathit{c}}$, a critical behavior is detected. A theoretical scheme for interpretation of phase transitions in smectic films is proposed and on its basis a conjecture on tension-induced phase transitions is formulated.