Difference in critical behaviour of the phase transitions nematic-smectic A1 and nematic-smectic Ad

Abstract

A high resolution X-ray study is presented of the nematic to smectic Aj phase transition in BOPCBOB and of the nematic to smectic Ad Phase transition in a binary mixture of 7CB and BCB (7.6CB). The known critical exponents of the heat capacity of both these systems agree with the predictions for the 3D XY model. For BOPCBOB we find values for the susceptibility y and the parallel correlation length vi that also agree with the 3D XY model, thus confirming the predictions of de Gennes. However, the values y and vi of 7.6CB are larger than the 3D XY values. This indicates a possible difference in the critical behaviour of the nematic- smectic Ai and the nematic-smectic Ad Phase transition. In all cases vi is larger than v~, Furthermore it is shown how the values for the critical exponents depend on the specific form chosen for the structure factor,