Attractive interactions, molecular complexes, and polarons in coupled dipolar exciton fluids

TL;DR

This paper theoretically investigates the interactions, binding energies, and polaron formation of dipolar excitons in stacked double quantum well bilayers, revealing how attractive and repulsive dipolar couplings influence exciton behavior and many-body effects.

Contribution

It introduces a microscopic model of dipolar exciton interactions in stacked quantum wells, analyzing how these interactions deform exciton wave functions and lead to polaron formation, with comparison to experimental data.

Findings

Attractive dipolar interactions deform exciton wave functions.

Interlayer binding energies decrease with exciton density in lattice models.

Single excitons induce polaron formation, affecting exciton distribution.

Abstract

Dipolar (or spatially indirect) excitons (IXs) in semiconductor double quantum well (DQW) subjected to an electric field are neutral species with a dipole moment oriented perpendicular to the DQW plane. Here, we theoretically study interactions between IXs in stacked DQW bilayers, where the dipolar coupling can be either attractive or repulsive depending on the relative positions of the particles. By using microscopic band structure calculations to determine the electronic states forming the excitons, we show that the attractive dipolar interaction between stacked IXs deforms their electronic wave function, thereby increasing the inter-DQW interaction energy and making the IX electrically polarizable. Many-particle effects interaction are addressed by considering the coupling between a single IX in one of the DQWs to a cloud of IXs in the other DQW, which is modeled either as a closed-packed lattice or as a continuum IX fluid. We find that the lattice model yields IX interlayer binding energies decreasing with increasing lattice density. This behavior is due to the dominating role of the intra-DQW dipolar repulsion, which prevents more than one exciton from entering the attractive region of the inter-DQW coupling. Finally, both models shows that the single IX distorts the distribution of IXs in the adjacent DQW, thus inducing the formation of an IX polaron. While the interlayer binding energy reduces with IX density for lattice polarons, the continuous polaron model predicts a non-monotonous dependence on density in semi-quantitative agreement with a recent experimental study [cf. Hubert {\it et al.}, Phys. Rev. {\bf X}9, 021026 (2019)].