Sliding phases in XY models, crystals, and cationic lipid-DNA complexes

We predict the existence of a new class of phases in weakly coupled, three- dimensional stacks of two-dimensional (2D) XY models. These "sliding phases " behave like decoupled, independent 2D XY models with zero free-energy cos t for macroscopic relative rotation of spins in different layers and algebr aic decay of two-point spin correlation functions with in-plane separation. Our results, which contradict past studies because we include higher-gradi ent couplings between layers, also apply to crystals and may explain recent ly observed behavior in cationic lipid-DNA complexes.