Gyroid symmetry and phase diagram of weak crystallization

The phase diagram in the (tau-delta) plane, where tau is the reduced temperature and delta is a parameter characterizing the anisotropy of the fourth vertex of the effective Hamiltonian, is constructed on the basis of the standard mean-field approximation of the theory of weak crystallization. It is shown that the region of the gyroid phase dominates the phase diagram for intermediate values of the parameters tau and delta. An expression is obtained for the free energy of the gyroid phase. (C) 1996 American Institute of Physics.