New insights into the nematic to smectic-A transition: X-ray measurements in a strong magnetic field

Abstract

The nematic to smectic-A (NA) phase transition would appear to be the simplest example of a freezing transition in nature. However, it has proven to be one of the most challenging problems of liquid crystal science and equilibrium statistical mechanics. One quantitative method to study this transition, that yields high quality data of almost transparent interpretation, has been X-ray measurements of the divergence of smectic order correlations as the SmA phase is approached from the nematic side. However, even data from these experiments have proved difficult to interpret because of the imperfect magnetic alignment or “mosaic spread” of the nematic phase. This mosaic spread limits the experimental transverse resolution and hence affects the data very close to the transition. Previous studies employed magnetic fields of ∼0.1–0.8 T. The use of a high (5 T) field, in the work we recently completed, improved the effective transverse resolution by almost two orders of magnitude allowing us to obtain results essentially free of mosaic spread. The generally used technique for removing this effect has now been tested. The high field results shed new light on this transition and reveal a new and proper way to apply the mosaicity correction.