A local Yang-Baxter obstruction for the hard-square model

Abstract

Many exact solutions of two-dimensional lattice models rest on an analytic family of weights satisfying the Yang-Baxter equation. The hard-hexagon model was solved in this way, while the neighbouring hard-square model has no known closed form. We determine the height alphabets that encode hard squares without hidden heights, and there are exactly two connected ones, the occupation variables themselves and the four heights of Andrews, Baxter and Forrester. Hard hexagons is integrable on both, but no analytic Yang-Baxter family of face weights passes through hard squares at nonzero activity on either. The difference lies in the exclusion rule, which for hard squares turns a few Yang-Baxter equations into polynomial identities that force auxiliary weights to vanish. A closed form for hard squares is not excluded, but a solution by this route would have to leave these encodings or go beyond a single analytic family through the model. Encodings with hidden heights remain open.