ORIENTATIONAL EPITAXY OF A CRYSTAL-E OVERLAYER ON A HEXATIC-B SUBSTRATE

Thermal and structural experiments on thin free-standing liquid-crystal films of n-heptyl-4'-n-pentyloxybiphenyl-4-carboxylate have revealed the existence of a surface-freezing phenomenon, in which surface crystal-E layers coexist with a hexatic-B interior. The crystal-E surface is typically found by electron diffraction to exist in twin domains in which the orientation of the orthorhombic lattice is rotated by 2-degrees-3-degrees with respect to that of the underlying hexatic-B bond-orientational axes. It is shown theoretically that such a rotation minimizes the energy of the strain caused by the two incommensurate types of structural order, analogous to the Novaco-McTague theory, which applies to the orientational epitaxy of a crystalline overlayer on an incommensurate crystalline substrate.