Coulomb-bound four- and five-particle valleytronic states in an atomically-thin semiconductor

TL;DR

This paper reports the experimental discovery of four- and five-particle valleytronic states in monolayer tungsten diselenide, revealing complex bound states with potential for quantum device applications.

Contribution

It provides the first experimental evidence of multi-particle valleytronic states involving dark excitons in atomically-thin semiconductors.

Findings

Detection of valleytronic four-particle biexcitons.

Observation of five-particle exciton-trions.

Large negative valley polarizations observed.

Abstract

As hosts for tightly-bound electron-hole pairs carrying quantized angular momentum, atomically-thin semiconductors of transition metal dichalcogenides provide an appealing platform for optically addressing the valley degree of freedom. In particular, the valleytronic properties of neutral and charged excitons in these systems have been widely investigated. Meanwhile, correlated quantum states involving more particles are so far elusive and controversial. Here we present experimental evidence for valleytronic four-particle biexcitons and five-particle exciton-trions in high-quality monolayer tungsten diselenide samples. Through charge doping, thermal activation, and magnetic-field tuning measurements, we determined that the biexciton and exciton-trion optical emissions are bound with respect to the bright exciton and the trion respectively. Further, both the biexciton and the exciton-trion are intervalley bound states involving dark excitons, giving rise to emissions with large, negative valley polarizations in contrast to that of the well-known two-particle excitons. Our studies provide new opportunities for building valleytronic quantum devices harnessing high-order excitations.