Anomalous temperature-dependent spin-valley polarization in monolayer WS$_{2}$

TL;DR

This study investigates the temperature-dependent spin-valley polarization in monolayer WS₂, revealing how trion emissions exhibit unique polarization behaviors influenced by multiparticle interactions and enabling potential control via gating.

Contribution

It demonstrates the temperature-dependent polarization of trion emissions in WS₂ and identifies the underlying multiparticle scattering mechanisms involved.

Findings

Trion emission shows 28% polarization at room temperature.

Temperature dependence linked to Coulomb-assisted intervalley scattering.

Gate voltages can modulate polarization and emission intensity.

Abstract

Single layers of transition metal dichalcogenides (TMDs) are direct gap semiconductors with nondegenerate valley indices. An intriguing possibility for these materials is the use of their valley index as an alternate state variable. Several limitations to such a utility include strong, phonon-enabled intervalley scattering, as well as multiparticle interactions leading to multiple emission channels. We prepare single-layer WS$_{2}$ such that the photoluminescence is from either the neutral or charged exciton (trion). After excitation with circularly polarized light, the neutral exciton emission has zero polarization, however, the trion emission has a large polarization (28%) at room temperature. The trion emission also has a unique, non-monotonic temperature dependence that we show is a consequence of the multiparticle nature of the trion. This temperature dependence enables us to determine that coulomb assisted intervalley scattering, electron-hole radiative recombination, and a 3-particle Auger process are the dominant mechanisms at work in this system. Because this dependence involves trion systems, one can use gate voltages to modulate the polarization (or intensity) emitted from TMD structures.