Delocalization of the height function of the six-vertex model

We show that the height function of the six-vertex model, in the parameter range $mathbf a=mathbf b=1$ and $mathbf cge1$, is delocalized with logarithmic variance when $mathbf cle 2$. This complements the earlier proven localization for $mathbf c>2$. Our proof relies on Russo--Seymour--Welsh type arguments, and on the local behaviour of the free energy of the cylindrical six-vertex model, as a function of the unbalance between the number of up and down arrows.