Nonlinear Sigma Models with Compact Hyperbolic Target Spaces

TL;DR

This paper investigates nonlinear sigma models with compact hyperbolic target spaces, revealing a topological phase transition and complex vortex behavior, which are studied through Monte Carlo simulations and linked to geometric properties.

Contribution

It introduces a novel class of sigma models with hyperbolic target spaces and analyzes their phase structure and vortex dynamics using computational methods.

Findings

Identified a topological phase transition at a critical temperature.

Observed proliferation of multiple vortex types above the transition.

Spins cluster around Weierstrass points below the critical temperature.

Abstract

We explore the phase structure of nonlinear sigma models with target spaces corresponding to compact quotients of hyperbolic space, focusing on the case of a hyperbolic genus-2 Riemann surface. The continuum theory of these models can be approximated by a lattice spin system which we simulate using Monte Carlo methods. The target space possesses interesting geometric and topological properties which are reflected in novel features of the sigma model. In particular, we observe a topological phase transition at a critical temperature, above which vortices proliferate, reminiscent of the Kosterlitz-Thouless phase transition in the $O(2)$ model. Unlike in the $O(2)$ case, there are many different types of vortices, suggesting a possible analogy to the Hagedorn treatment of statistical mechanics of a proliferating number of hadron species. Below the critical temperature the spins cluster around six special points in the target space known as Weierstrass points. The diversity of compact hyperbolic manifolds suggests that our model is only the simplest example of a broad class of statistical mechanical models whose main features can be understood essentially in geometric terms.