Interlayer exciton valley polarization dynamics in large magnetic fields

TL;DR

This study investigates how magnetic fields influence the valley polarization dynamics of interlayer excitons in van der Waals heterostructures, revealing distinct behaviors depending on the stacking alignment type.

Contribution

It provides the first detailed analysis of valley polarization dynamics of interlayer excitons in large magnetic fields for different stacking alignments.

Findings

PL circular degree of polarization increases and saturates near unity in H-type heterostructures.

PL polarization decreases and crosses zero in R-type heterostructures before saturating with opposite polarization.

Distinct behaviors are explained by a model considering ILE states and their selection rules.

Abstract

In van der Waals heterostructures (HS) consisting of stacked MoSe$_2$ and WSe$_2$ monolayers, optically bright interlayer excitons (ILE) can be observed when the constituent layers are crystallographically aligned. The symmetry of the monolayers allows for two different types of alignment, in which the momentum-direct interlayer transitions are either valley-conserving (R-type alignment) or changing the valley index (H-type anti-alignment). Here, we study the valley polarization dynamics of ILE in magnetic fields up to 30~Tesla by time-resolved photoluminescence (PL). For all ILE types, we find a finite initial PL circular degree of polarization ($DoP$) after unpolarized excitation in applied magnetic fields. For ILE in H-type HS, we observe a systematic increase of the PL $DoP$ with time in applied magnetic fields, which saturates at values close to unity for the largest fields. By contrast, for ILE in R-type HS, the PL $DoP$ shows a decrease and a zero crossing before saturating with opposite polarization. This unintuitive behavior can be explained by a model considering the different ILE states in H- and R-type HS and their selection rules coupling PL helicity and valley polarization.