Classification of (frustrated) 2D Ising models in genus 1

Abstract: We prove a complete classification of all 2D Ising models, frustrated or not, whose underlying spectral curve has genus 1. As a specific case, we recover Baxter's $Z$-invariant Ising model, thus extending his class of models to \emph{real} coupling constants. We identify two additional families of models, both having \emph{non}-Harnack spectral curves. We show that in all cases the spectral curve is maximal. Moreover, each family undergoes an algebraic phase transition as the genus of the curve tends to 0, explaining the different behaviors observed in the physics literature. In our proof, we use properties of the spectral curve, and Fock's approach. This yields a natural framework for a further systematic study of the frustrated Ising model, in particular for proving local formulas. As an example of application, we prove a full classification of the frustrated Ising model on the triangular lattice.