Collective interlayer pairing and pair superfluidity in vertically stacked layers of dipolar excitons

TL;DR

This paper uses quantum Monte Carlo simulations to explore collective interlayer pairing and pair superfluidity in stacked dipolar exciton layers, revealing nonmonotonic energy shifts and cascade BKT transitions relevant to experimental observations.

Contribution

It provides a theoretical analysis of interlayer pairing signatures and superfluid transitions in dipolar exciton layers, guiding future experimental detection.

Findings

Nonmonotonic energy shifts with density imbalance

Discontinuous jump at interlayer molecule gap

Cascade of BKT transitions into pair superfluidity

Abstract

Layered bosonic dipolar fluids have been suggested to host a condensate of interlayer molecular bound states. However, its experimental observation has remained elusive. Motivated by two recent experimental works [Hubert et al., Phys. Rev. X 9, 021026 (2019) and D. J. Choksy et al., Phys. Rev. B 103 045126 (2021)], we theoretically study, using numerically exact quantum Monte Carlo calculations, the experimental signatures of collective interlayer pairing in vertically stacked indirect exciton (IX) layers. We find that IX energy shifts associated with each layer evolve non trivially as a function of density imbalance following a nonmonotonic trend with a jump discontinuity at density balance, identified with the interlayer IX molecule gap. This behavior discriminates between the superfluidity of interlayer bound pairs and independent dipole condensation in distinct layers. Considering finite temperature and finite density imbalance conditions, we find a cascade of Berezinskii--Kosterlitz--Thouless (BKT) transitions, initially into a pair superfluid and only then, at lower temperatures, into complete superfluidity of both layers. Our results may provide a theoretical interpretation of existing experimental observations in GaAs double quantum well (DQW) bilayer structures. Furthermore, to optimize the visibility of pairing dynamics in future studies, we present an analysis suggesting realistic experimental settings in GaAs and transition metal dichalcogenide (TMD) bilayer DQW heterostructures where collective interlayer pairing and pair superfluidity can be clearly observed.