Absence of vortex condensation in a two dimensional fermionic XY model

TL;DR

This paper constructs a 2D fermionic model with U(1) symmetry, demonstrating numerically that it lacks a Kosterlitz-Thouless transition and vortex condensation, and shows how multilayering can induce such a transition.

Contribution

It introduces a fermionic lattice model with U(1) symmetry that does not exhibit vortex condensation or KT transition, contrasting with the XY model.

Findings

The model remains in a gapless phase without vortex condensation.

Numerical evidence shows absence of KT transition in the model.

Adding layers can induce vortex condensation and a KT transition.

Abstract

Motivated by a puzzle in the study of two dimensional lattice Quantum Electrodynamics with staggered fermions, we construct a two dimensional fermionic model with a global U(1) symmetry. Our model can be mapped into a model of closed packed dimers and plaquettes. Although the model has the same symmetries as the XY model, we show numerically that the model lacks the well known Kosterlitz-Thouless phase transition. The model is always in the gapless phase showing the absence of a phase with vortex condensation. In other words the low energy physics is described by a non-compact U(1) field theory. We show that by introducing an even number of layers one can introduce vortex condensation within the model and thus also induce a KT transition.