Squareness for the Monopole-Dimer model

TL;DR

This paper proves that the partition function of the monopole-dimer model on certain symmetric graphs is a perfect square and introduces a new combinatorial model called dicots, extending the model's applicability and providing new computational tools.

Contribution

It establishes the squareness of the monopole-dimer partition function on involution-invariant graphs and introduces dicots, a new graph type, for combinatorial interpretation and determinant formulation.

Findings

Partition function is a perfect square for involution-invariant graphs.

Introduces dicots, a new graph structure with two edge types.

Provides determinant formula for the partition function of dicots.

Abstract

The monopole-dimer model introduced recently is an exactly-solvable signed generalisation of the dimer model. We show that the partition function of the monopole-dimer model on a graph invariant under a fixed-point free involution is a perfect square. We give a combinatorial interpretation of the square-root of the partition function for such graphs in terms of a monopole-dimer model on a new kind of graph with two types of edges which we call a dicot. The partition function of the latter can be written as a determinant, this time of a complex adjacency matrix. This formulation generalises T. T. Wu's assignment of imaginary orientation for the grid graph to planar dicots. As an application, we compute the partition function for a family of non-planar dicots with positive weights.