EVIDENCE FOR A NEMATIC-NEMATIC TRANSITION IN THIN CELLS OF A HIGHLY POLAR COMPOUND

The report experimental evidence for a weak first-order nematic-nemati c transition in thin (similar to 1-2 mu m) cells of the strongly polar nematogen, p-cyanophenyl p-n heptylbenzoate. The transition temperatu re T-NN increases as the thickness of the cell decreases indicating th at, in the bulk, this transition may occur at temperatures well below the ambient one. Optical and dielectric measurements of the order para meters in thin cells show that there is considerable enhancement compa red to the bulk values. T-NN can be increased significantly by moderat e electric and magnetic fields. The transition probably indicates a ju mp in the concentration of neighbours with antiparallel configurations compared to those with parallel configurations. The results imply tha t the order parameter coherence length in the, compound is relatively large.