Discrete interactions between a few interlayer excitons trapped at a MoSe2–WSe2 heterointerface

Abstract

Inter-layer excitons (IXs) in hetero-bilayers of transition metal dichalcogenides (TMDs) represent an exciting emergent class of long-lived dipolar composite bosons in an atomically thin, near-ideal two-dimensional (2D) system. The long-range interactions that arise from the spatial separation of electrons and holes can give rise to novel quantum, as well as classical multi-particle correlation effects. Indeed, first indications of exciton condensation have been reported recently. In order to acquire a detailed understanding of the possible many-body effects, the fundamental interactions between individual IXs have to be studied. Here, we trap a tunable number of dipolar IXs (NIX ~ 1–5) within a nanoscale confinement potential induced by placing a MoSe2–WSe2 hetero-bilayer (HBL) onto an array of SiO2 nanopillars. We control the mean occupation of the IX trap via the optical excitation level and observe discrete sharp-line emission from different configurations of interacting IXs. The intensities of these features exhibit characteristic near linear, quadratic, cubic, quartic and quintic power dependencies, which allows us to identify them as different multiparticle configurations with NIX ~ 1–5. We directly measure the hierarchy of dipolar and exchange interactions as NIX increases. The interlayer biexciton (NIX = 2) is found to be an emission doublet that is blue-shifted from the single exciton by ΔE = (8.4 ± 0.6) meV and split by 2J = (1.2 ± 0.5) meV. The blueshift is even more pronounced for triexcitons ((12.4 ± 0.4) meV), quadexcitons ((15.5 ± 0.6) meV) and quintexcitons ((18.2 ± 0.8) meV). These values are shown to be mutually consistent with numerical modelling of dipolar excitons confined to a harmonic trapping potential having a confinement lengthscale in the range $$\ell \approx 3$$ ℓ ≈ 3 nm. Our results contribute to the understanding of interactions between IXs in TMD hetero-bilayers at the discrete limit of only a few excitations and represent a key step towards exploring quantum correlations between IXs in TMD hetero-bilayers.

Optical read-out of Coulomb staircases in a moiré superlattice via trapped interlayer trions

“…Further, the spatial mapping of the trapped IX positions indicates that, at the exceptionally low density of excitons that we optically generated, their position is essentially random: the moiré lattice is not discernible. We speculate that the particular site chosen by the exciton is likely determined by the local environment and perhaps aided by dipolar repulsion effects 31 , 32 .…”

Interlayer exciton formation, relaxation, and transport in TMD van der Waals heterostructures

“…From the linear fit of the energy shift versus electric field, the dipole size or charge separation ( d ) has been estimated to be ~0.5–0.8 nm, matching well with the expected layer separation (~0.7 nm) 30 , 95 , 117 . Another consequence of the static electric dipole is the resulting dipole–dipole repulsive interactions between the interlayer excitons 22 , 28 , 30 , 118 , 119 , as revealed by a blue-shift in the PL energy with increasing exciton density (excitation power) (Fig. 3e ) 22 , 28 , 30 .…”

Self-trapped interlayer excitons in van der Waals heterostructures

“…For example, TMDC bilayer heterostructures with twist-angle induces spatially periodic moiré superlattice potential that can trap IX, which has been predicted by several theoretical studies [23][24][25][26][27][28] and confirmed by a series of experiments [5,[29][30][31][32][33][34][35]. Strain has also been proposed as an effective way to trap IX, in which the band structures of TMDC heterostructures are modified by strain, resulting in the static potential well [16,[36][37][38]. The spatially varying electric fields enable the localization of IX due to its permanent dipole moment [10,39,40], where the deterministic placement and control of a single trapped IX has been achieved [39].…”