Disordered cold atoms in different symmetry classes

TL;DR

This paper investigates how disorder affects non-interacting cold atoms in a 2D optical lattice across four symmetry classes, revealing phenomena like domain walls, vortices, and extended states depending on the class and disorder strength.

Contribution

It provides a comprehensive analysis of disorder effects in four symmetry classes of cold atoms, including the emergence of topological excitations and localization behavior.

Findings

Chiral classes support a metallic phase near zero energy.

Excitations manifest as domain walls or vortices depending on symmetry.

Eigenstates in non-chiral models remain extended at higher disorder levels.

Abstract

We consider an experimentally realizable model of non-interacting but randomly coupled atoms in a two-dimensional optical lattice. By choosing appropriate real or complex-valued random fields and species-dependent energy offsets, this system can be used to analyze effects of disorder in four different classes: The chiral BDI and AIII, and the A and AI symmetry classes. These chiral classes are known to support a metallic phase at zero energy, which here, due to the inevitable finite size of the system, should also persist in a neighborhood of non-zero energies. As we discuss, this is of particular interest for experiments involving quenches. Away from the centre of the spectrum, we find that excitations appear as domain walls in the cases with time-reversal symmetry, or as vortices in the cases where time-reversal symmetry is absent. Therefore, a quench in a system with uniform density would lead to the formation of either vortices or domain walls depending on the symmetry class. For the non-chiral models in the A and AI classes, a population imbalance between the two atomic species naturally occurs. In these cases, one of the two species is seen to favour a more uniform density. We also study the onset of localization as the disorder strength is increased for the different classes, and by deriving an effective model for the non-chiral cases we show how their eigenstates remain extended for larger values of the coupling with the disorder, if compared to the non-chiral ones.