FOCUS 

Electron diffraction is used to study the structure of free-standing smectic liquid-crystal films as they undergo novel phase transitions. This unique experimental approach has made it possible to measure quantitatively the bond-orientational and/or crystalline order within single domains in films of a few molecular layers for both tilted and untilted phases. The technique involves the use of an electron microscope with a differentially pumped, temperature-controlled sample chamber and an electron beam as small as a few microns in diameter. The properties near the crystal-hexatic, hexatic-liquid, and crystal-liquid phase transitions are examined. Special attention is paid to the evolving structure of the surface of the films in relation to that of the interior as the transitions take place. The results are expected to yield valuable experimental data to provide additional insight about the details of the melting process and the associated wetting behavior of the interface for comparison with the theoretical predictions.

SELECTED PROJECTS

• Multiple-step melting transitions in two-dimensional liquid-crystal films
• Scaling of 6n-fold bond-orientational order parameters in a hexatic liquid crystal
• Nature of layer-by-layer freezing in free-standing liquid-crystal films
• Phase transitions in tilted smectic liquid-crystal films

PUBLICATIONS

• C. F. Chou, A. J. Jin, S. W. Hui, C. C. Huang, and J. T. Ho. Multiple-step melting in two-dimensional liquid-crystal films. Science 280:1424 (1998).
• C. Y. Chao, S. W. Hui, J. E. Maclennan, C. F. Chou, and J. T. Ho. Surface-freezing transitions and novel tilted hexatic phases in smectic liquid-crystal films. Phys. Rev. Lett. 78:2581 (1997).
• C. Y. Chao, S. W. Hui, and J. T. Ho. Layer-dependent surface-induced tilt, hexatic, and positional orders in free-standing 7O.7 films. Phys. Rev. Lett. 78:4962 (1997).
• C. F. Chou, J. T. Ho, S. W. Hui, and V. Surendranath. Scaling of 6n-fold bond-orientational order parameters in a hexatic liquid-crystal thin film. Phys. Rev. Lett. 76:4556 (1996).
• C. Y. Chao, C. F. Chou, J. T. Ho, S. W. Hui, A. J. Jin, and C. C. Huang. Nature of layer-by-layer freezing in free-standing 4O.8 films. Phys. Rev. Lett. 77:2750 (1996).

Fig 1 Electron diffraction patterns from a nine-layer 54COOBC film:
(a) in the hexatic-B phase

(b) in the crystal-B phase.