Sliding Phases in XY-Models, Crystals, and Cationic Lipid-DNA Complexes

TL;DR

This paper predicts a new class of phases called sliding phases in weakly coupled 3D stacks of 2D XY-models, characterized by decoupled layers with algebraic correlations, and explains their relevance to crystals and lipid-DNA complexes.

Contribution

It introduces the concept of sliding phases in 3D stacks of 2D XY-models, including higher-gradient couplings, and connects these phases to experimental observations in related systems.

Findings

Existence of sliding phases with decoupled layers

Algebraic decay of in-plane spin correlations

Relevance to crystal behavior and lipid-DNA complexes

Abstract

We predict the existence of a totally new class of phases in weakly coupled, three-dimensional stacks of two-dimensional (2D) XY-models. These ``sliding phases'' behave essentially like decoupled, independent 2D XY-models with precisely zero free energy cost associated with rotating spins in one layer relative to those in neighboring layers. As a result, the two-point spin correlation function decays algebraically with in-plane separation. Our results, which contradict past studies because we include higher-gradient couplings between layers, also apply to crystals and may explain recently observed behavior in cationic lipid-DNA complexes.