Square lattice Ising model susceptibility: connection matrices and singular behavior of $\chi^{(3)}$ and $\chi^{(4)}$

TL;DR

This paper introduces an efficient method to compute connection matrices for large Fuchsian differential equations related to the Ising model's susceptibility, revealing critical behaviors and monodromy structures of $\,\chi^{(3)}$ and $\,\chi^{(4)}$.

Contribution

It develops a novel approach to calculate connection matrices and monodromy matrices for high-order Fuchsian equations in the Ising model susceptibility analysis.

Findings

Confirmed non-singularity of quadratic number singularities for $\,\chi^{(3)}$.

Derived exact monodromy matrices for $\,\chi^{(3)}$ and $\,\chi^{(4)}$.

Identified qualitative differences in correction-to-scaling behaviors.

Abstract

We present a simple, but efficient, way to calculate connection matrices between sets of independent local solutions, defined at two neighboring singular points, of Fuchsian differential equations of quite large orders, such as those found for the third and fourth contribution ($\chi^{(3)}$ and $\chi^{(4)}$) to the magnetic susceptibility of square lattice Ising model. We deduce all the critical behaviors of the solutions $\chi^{(3)}$ and $\chi^{(4)}$, as well as the asymptotic behavior of the coefficients in the corresponding series expansions. We confirm that the newly found quadratic number singularities of the Fuchsian ODE associated to $\chi^{(3)}$ are not singularities of the particular solution $\chi^{(3)}$ itself. We use the previous connection matrices to get the exact expressions of all the monodromy matrices of the Fuchsian differential equation for $\chi^{(3)}$ (and $\chi^{(4)}$) expressed in the same basis of solutions. These monodromy matrices are the generators of the differential Galois group of the Fuchsian differential equations for $\chi^{(3)}$ (and $\chi^{(4)}$), whose analysis is just sketched here. As far as the physics implications of the solutions are concerned, we find challenging qualitative differences when comparing the corrections to scaling for the full susceptibillity $\chi$ at high temperature (resp. low temperature) and the first two terms $\chi^{(1)}$ and $\chi^{(3)}$ (resp. $\chi^{(2)}$ and $\chi^{(4)}$) .