$\Gamma$-convergence analysis of a generalized $XY$ model: fractional vortices and string defects

TL;DR

This paper analyzes a generalized 2D XY model with multiple wells, deriving a continuum limit that captures fractional vortices and string defects, relevant to various physical phenomena.

Contribution

It introduces a novel generalized XY model with multiple wells and derives its Gamma-limit, capturing fractional vortices and string defects in the continuum.

Findings

Gamma-limit includes renormalized energy for vortices

Surface energy proportional to string length

Model describes topological defects in physics and materials science

Abstract

We propose and analyze a generalized two dimensional $XY$ model, whose interaction potential has $n$ weighted wells, describing corresponding symmetries of the system. As the lattice spacing vanishes, we derive by $\Gamma$-convergence the discrete-to-continuum limit of this model. In the energy regime we deal with, the asymptotic ground states exhibit fractional vortices, connected by string defects. The $\Gamma$-limit takes into account both contributions, through a renormalized energy, depending on the configuration of fractional vortices, and a surface energy, proportional to the length of the strings. Our model describes in a simple way several topological singularities arising in Physics and Materials Science. Among them, disclinations and string defects in liquid crystals, fractional vortices and domain walls in micromagnetics, partial dislocations and stacking faults in crystal plasticity.