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.