Topological Varma superfluid in optical lattices

TL;DR

This paper proposes a new interaction-driven topological state of bosons in an optical lattice, where interactions induce an anomalous Hall effect in excitations despite the non-topological single-particle spectrum.

Contribution

It demonstrates the emergence of a topological state driven solely by interactions in a bosonic system, specifically in a Lieb optical lattice with ultracold bosons.

Findings

Interactions induce an anomalous Hall effect for excitations.

The system exhibits a non-zero Chern number for excitations.

The single-particle spectrum is non-topological, but interactions create topological features.

Abstract

Topological states of matter are peculiar quantum phases showing different edge and bulk transport properties connected by the bulk-boundary correspondence. While non-interacting fermionic topological insulators are well established by now and have been classified according to a ten-fold scheme, the possible realisation of topological states for bosons has not been much explored yet. Furthermore, the role of interactions is far from being understood. Here, we show that a topological state of matter exclusively driven by interactions may occur in the p-band of a Lieb optical lattice filled with ultracold bosons. The single-particle spectrum of the system displays a remarkable parabolic band-touching point, with both bands exhibiting non-negative curvature. Although the system is neither topological at the single-particle level, nor for the interacting ground state, on-site interactions induce an anomalous Hall effect for the excitations, carrying a non-zero Chern number. Our work introduces an experimentally realistic strategy for the formation of interaction-driven topological states of bosons.