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Proc. Nati. Acad. Sci. USA Vol. 84, p. 4689, July 1987 Symposium Paper

This paper was presented at a symposium "Interfaces and Thin Films," organized by John Armstrong, Dean E. Eastman, and George M. Whitesides, held March 23 and 24, 1987, at the National Academy of Sciences, Washington, D.C. , , and crystals ROBERT J. BIRGENEAU Department of , Massachusetts Institute of Technology, Cambridge, MA 02139

It has been known for many decades that condensed long-range BO order and short-range positional order as systems may be described by independent positional and predicted in refs. 2 and 3. Further, from a quantitative bond orientational (BO) order parameters (1). Further, there analysis of the diffraction profiles for the thick films it is is no symmetry reason why the positional and BO order must possible to deduce the behavior of the BO order parameter vanish at the same . Thus it is possible to have a and eight higher harmonics in the angular structure. Stimu- of matter in which the positional order is short range lated by these experiments, we have developed a theory for as in a liquid or but the BO order is long range as in a the growth of hexatic phases in three dimensions (11). As we crystalline . Indeed, by generalizing concepts developed shall discuss, the theory is successful in part, but there are in the context of theories of two-dimensional dislocation- some remaining uncertainties. Experiments on very thin mediated , it has been suggested that such an unusual films are at an earlier stage of development (refs. 4-6; J. D. phase could occur in certain smectic liquid Brock, D. Y. Noh, B. McLain, J. D. Litster, and R.J.B., (2, 3). This phase has been labeled "stacked hexatic" for unpublished work). We have managed to produce stable hexagonal symmetry systems. Pindak and co-workers (4, 5) four-layer films and to study the development of the posi- made the early observations of this novel phase in freely tional and orientational order. However, further experiments suspended films (for thin-film SF and SI phases, are required to characterize properly the differences in see ref. 6). However, in the materials studied by that group behavior in two- and three-dimensional systems. Indepen- it was not possible to produce single-domain samples, so the dent optical studies by Dierker and co-workers (12) have BO order could not be studied quantitatively. revealed unique textural defects in two-layer hexatic liquid Very interesting theoretical predictions have been made crystal films. We will discuss these results and their relation- for the smectic liquid crystal phases, SC, SF, and SI, in which ship to our x-ray scattering studies. the are tilted with respect to the layer normal (7, 8). SF and SI are hexatic phases with molecules tilted between This work was supported by the Joint Services Electronics Program under Contract DAAL03-86-K-0002 and by the National and towards near neighbors, respectively. However, cou- Science Foundation Materials Research Laboratory under Grant pling to the molecular tilt should induce long-range hexatic DMR84-18718. order even in the Sc phase, This prediction by itself presents an interesting problem for study. An equally attrac- 1. Landau, L. D. & Lifshitz, E. M (1969) tive feature of the tilted hexatics is that the coupling between (Addison-Wesley, Reading, MA), p. 466. the tilt and BO order provides a possible avenue for the 2. Birgeneau, R. J. & Litster, J. D. (1978) J. Phys. Lett. 39, growth of single-domain samples. This would, in turn, make L-399-L-402. 3. Halperin, B. I. & Nelson, D. R. (1978) Phys. Rev. Lett. 41, possible direct measurements of the BO order, an essential 121-124. aspect of BO ordered phases which has not yet been studied 4. Pindak, A., Moncton, D. E., Davey, S. C. & Goodby, J. W. in substrate-free systems. (1981) Phys. Rev. Lett. 46, 1135-1138. We have performed a series of synchrotron x-ray diffrac- 5. Budai, J., Pindak, R., Davey, S. C. & Goodby, J. W. (1984) J. tion measurements on free-standing liquid crystal films ofthe Phys. Lett. 45, L-1053-L-1062. racemic 4-(2-methylbutyl)phenyl-4'-octyloxy-1,1'- 6. Sirota, E. B., Pershan, P. S., Sorensen, L. B. & Collettj J. -4-carboxylate (80SI). This system is believed to (1985) Phys. Rev. Lett. 55, 2039-2042. exhibit a continuous Sc-SI transition (9). Experiments have 7. Nelson, D. R. & Halperin, B. I. (1980) Phys. Rev. B: Condens. been carried out for films whose thickness varied from 10 pim Matter 21, 5312-5329. 122 A J. D. D. Y. B. 8. Bruinsma, R. & Nelson, D. R. (1981) Phys. Rev. B: Condens. to (four layers) (ref. 10; Brock, Noh, Matter 23, 402-410. McLain, J. D. Litster, and R.J.B., unpublished work). By 9. Gane, P. A. C., Leadbetter, A. J. & Wrighton, P. G. (1981) cooling slowly in a transverse magnetic field it is indeed Mol. Cryst. Liq. Cryst. 66, 247-266. possible to produce single domains of the SI phase. We have 10. Brock, J. D., Aharony, A., Birgeneau, R. J., Evans-Lutterodt, confirmed that for both thin and thick films the SI phase has K. W., Litster, J. D., Horn, P. M., Stephenson, G. B. & Tajbakhsh, A. R. (1986) Phys. Rev. Lett. 57, 98-101. 11. Aharony, A., Birgeneau, R. J., Brock, J. D. & Litster, J. D. The publication costs of this article were defrayed in part by page charge (1986) Phys. Rev. Lett. 57, 1012-1015. payment. This article must therefore be hereby marked "advertisement" 12. Dierker, S. B., Pindak, R. & Meyer, R. B. (1986) Phys. Rev. in accordance with 18 U.S.C. §1734 solely to indicate this fact. Lett. 56, 1819-1822.

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