University of Minnesota
School of Physics & Astronomy

Physics and Astronomy Calendar

Tuesday, July 10th 2007
10:15 am:
THESIS DEFENSE in Physics, Room 358B
Speaker: Zengqiang (John) Liu
Subject: Optical and resonant x-ray studies of chiral smectic-C liquid crystals structures and transitions

Applying resonant x-ray diffraction and differential optical reflectivity, we determined that the phase transition between the SmC phase and SmC_alpha phase is a first-order transition that ends at a critical point. We reported observation of the critical point in binary mixtures of homologous liquid crystal compounds. We also determined the order parameter and two critical exponents associated with the transition. Comparison with the mean-field calculations suggested that long-range interactions are present in the critical region of the transition. With the same experimental probes, we studied the pitch evolution with temperature in the SmC_alpha phase near 4 layers. A theoretical model developed by Olson et al. predicted that the SmC_FI2 phase intervenes in the Sm_alpha phase where the pitch is near 4 layers. Our observation showed that the pitch in the SmC_alpha phase decreased continuously and strictly across 4 layers upon cooling while the SmC_FI2 phase appeared at a lower temperature than the SmC_alpha phase. We also found linear relation between the pitch and the layer spacing, which was not observed in other systems. Comparison with a newer theoretical model by Hameneh and Taylor suggested the presence of long-rang interactions in smectic liquid crystals.

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