Valley polarization of chiral excitonic bound states induced by band geometry

TL;DR

This paper investigates how band geometry and Berry phase influence excitonic pairing in layered van der Waals materials, revealing the emergence of chiral excitonic states and complex angular momentum mixing.

Contribution

It introduces a novel analysis of Berry flux effects on excitonic pairing, including chiral states and angular momentum mixing in multilayer graphene.

Findings

Finite angular momentum excitons are favored in certain parameter regimes.

Berry flux causes angular momentum states to cross, unlike in hydrogen in a magnetic field.

Multiple angular momenta mix in the ground state when rotational symmetry is broken.

Abstract

Van der Waals (vdW) materials provide a rich platform for exploring the interplay of interactions, topology, and paired-electron phases. We study how the Berry phase reshapes excitonic pairing in a double-well dispersion representative of layered vdW systems. By computing the temperature-versus-Berry flux phase diagram of the system, we find parameter ranges where finite angular momentum excitons are favored, including chiral states. Strikingly, the condensed angular momentum channel evolves with Berry flux, revealing a pairing problem with no analogue in a hydrogen atom in a uniform magnetic field, where angular momentum states never cross. We then turn to a model of multilayer rhombohedral graphene and examine the effects of trigonal warping. Once continuous rotational symmetry is broken, excitons mix multiple angular momenta, and for a range of parameters we find a variety of linear combination of angular momenta ($s, p, f$ and $g$) in the ground state. Our results point to the possibility of chiral excitonic condensates, and spontaneous symmetry breaking through many-body condensation.