Bond orientational order model for smectic B liquid crystals

We show how recent theories of two dimensional melting can be carried over to provide natural order parameters for solid, smectic B, and smectic A phases of liquid crystals. Smectic B and other well-ordered smectic phases correspond to systems with bond orientational long range order and positional short range order in the plane of the smectic layers. The X-ray scattering predicted by this model is discussed in detail.