Large photoluminescence enhancement by an out-of-plane magnetic field in exfoliated WS$_2$ flakes

TL;DR

This study demonstrates significant photoluminescence enhancement in WS2 flakes under an out-of-plane magnetic field at low temperature, revealing new insights into valleytronic behavior and trion properties.

Contribution

It introduces the first observation of out-of-plane magnetic field induced PL enhancement in WS2 and proposes mechanisms involving valley energy differences and electron-hole overlap.

Findings

Large PL enhancement at 4 K with out-of-plane magnetic field

Reduced Landé g factor for trions compared to excitons

Model explaining the smaller g factor of Λ-K trions

Abstract

We report an out-of-plane magnetic field induced large photoluminescence enhancement in WS${}_2$ flakes at $4$ K, in contrast to the photoluminescence enhancement provided by in-plane field in general. Two mechanisms for the enhancement are proposed. One is a larger overlap of electron and hole caused by the magnetic field induced confinement. The other is that the energy difference between $\Lambda$ and K valleys is reduced by magnetic field, and thus enhancing the corresponding indirect-transition trions. Meanwhile, the Land\'e g factor of the trion is measured as $-0.8$, whose absolute value is much smaller than normal exciton, which is around $|-4|$. A model for the trion g factor is presented, confirming that the smaller absolute value of Land\'e g factor is a behavior of this $\Lambda$-K trion. By extending the valley space, we believe this work provides a further understanding of the valleytronics in monolayer transition metal dichalcogenides.