Polar helical smectic
= Polar helical smectic
A polar helical smectic has a vector order parameter of constant magnitude whose direction rotates at a nonzero wavevector. For an isotropic Brazovskii free energy, all constant rotations of the helical plane are degenerate until a longitudinal-gradient term such as $|\nabla\mathbin\cdot\mathbf p|^2$ selects its orientation.