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°3°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 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.