  We consider a gradient interface model on the lattice with interaction
potential which is a non-convex perturbation of a convex potential. We show
using a one-step multiple scale analysis the strict convexity of the surface
tension at high temperature. This is an extension of Funaki and Spohn's result,
where the strict convexity of potential was crucial in their proof that for
every tilt there is a unique, shift invariant, ergodic Gibbs measure for the
$\nabla\phi$ field.
