  We present a phenomenological theory of phase transitions in achiral lipid
membranes in terms of two coupled order parameters -- a scalar order parameter
describing lipid chain melting, and a vector order parameter describing the
tilt of the hydrocarbon chains below the chain-melting transition. Existing
theoretical models fail to account for all the observed features of the phase
diagram, in particular the detailed microstructure of the asymmetric ripple
phase lying between the fluid and the tilted gel phase. In contrast, our
two-component theory reproduces all the salient structural features of the
ripple phase, providing a unified description of the phase diagram and
microstructure.
