Gradient Gibbs measures with non-convex potentials and the universality class of the Gaussian Free Field

We study a general class of gradient interface models with Hamiltonian $H=β\sum V(\nablaϕ)$, $β>0$, assuming essentially that the potential $V$ is even, $V'(s)\ge αs$ on $[0,\infty)$ for some $α>0$, and $-M\leq V''\le C$. We establish a Helffer-Sjöstrand representation for these models, and use it to prove that their scaling limits are Gaussian Free Fields (GFFs), and that their covariances decay at the same rate as the GFF. This extends results for strictly convex potentials to a large class of non-convex potentials and to arbitrary temperatures. Additionally, we prove Brascamp-Lieb and dimension-free Poincaré inequalities for the models. We obtain these results by representing the interface as a mixture of gradient interface models with strictly convex potentials, extending an idea by Biskup-Spohn who had considered mixtures of Gaussians at moderate inverse temperature $β=1$. The construction of such a representation is one of the key new contributions of this work.