Absence of topological order in the $U(1)$ checkerboard toric code

TL;DR

This paper demonstrates that the $U(1)$ checkerboard toric code lacks topological order across its entire parameter range, with finite-size gaps explained by geometric constraints and perturbation theory.

Contribution

The study provides a perturbative analysis showing the absence of topological order in the $U(1)$ checkerboard toric code, clarifying previous conjectures based on quantum Monte Carlo simulations.

Findings

Ground-state degeneracy is lifted in fourth-order perturbation theory.

Finite-size gaps are due to geometric constraints and are not indicative of topological order.

The system exhibits confined fracton excitations that are trivial in the thermodynamic limit.

Abstract

We investigate the $U(1)$ checkerboard toric code which corresponds to the $U(1)$-symmetry enriched toric code with two distinct star sublattices. One can therefore tune from the limit of isolated stars to the uniform system. The uniform system has been conjectured to possess non-Abelian topological order based on quantum Monte Carlo simulations suggesting a non-trivial ground-state degeneracy depending on the compactification of the finite clusters. Here we show that these non-trivial properties can be naturally explained in the perturbative limit of isolated stars. Indeed, the compactification dependence of the ground-state degeneracy can be traced back to geometric constraints stemming from the plaquette operators. Further, the ground-state degeneracy is fully lifted in fourth-order degenerate perturbation theory giving rise to a non-topological phase with confined fracton excitations. These fractons are confined for small perturbations so that they cannot exist as single low-energy excitation in the thermodynamic limit but only as topologically trivially composite particles. However, the confinement scale is shown to be surprisingly large so that finite-size gaps are extremely small on finite clusters up to the uniform limit which is calculated explicitly by high-order series expansions. Our findings suggest that these gaps were not distinguished from finite-size effects by the recent quantum Monte Carlo simulation in the uniform limit. All our results therefore point towards the absence of topological order in the $U(1)$ checkerboard toric code along the whole parameter axis.