Gapping fragile topological bands by interactions

TL;DR

This paper demonstrates that strong interactions can gap fragile topological bands without symmetry breaking, leading to featureless insulators, by forming trions that enable the gap opening.

Contribution

It introduces a mechanism where interactions induce a gap in fragile topological bands via trion formation, without breaking symmetries or topological order.

Findings

Interactions can open a gap in fragile topological bands at half filling.

The resulting ground state is not topologically ordered.

Trion formation enables the gap without symmetry breaking.

Abstract

We consider the stability of fragile topological bands protected by space-time inversion symmetry in the presence of strong electron-electron interactions. At the single-particle level, the topological nature of the bands prevents the opening of a gap between them. In contrast, we show that when the fragile bands are half filled, interactions can open a gap in the many-body spectrum without breaking any symmetry or mixing degrees of freedom from remote bands. Furthermore, the resulting ground state is not topologically ordered. Thus, a fragile topological band structure does not present an obstruction to forming a "featureless insulator" ground state. Our construction relies on the formation of fermionic bound states of two electrons and one hole, known as "trions". The trions form a band whose coupling to the electronic band enables the gap opening. This result may be relevant to the gapped state indicated by recent experiments in magic angle twisted bilayer graphene at charge neutrality.