Large Photoluminescence Enhancement by an Out-of-Plane Magnetic Field in Exfoliated WS_2 Flakes

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 an in-plane field in general. Two mechanisms for the enhancement are proposed. One is a larger overlap of the electron and hole caused by the magnetic field induced confinement. The other is that the energy difference between \varLambda and K valleys is reduced by magnetic field, and thus enhancing the corresponding indirect-transition trions. Meanwhile, the Landé g factor of the trion is measured to be -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 the Landé g factor is a behavior of this \varLambda–K trion. By extending the valley space, we believe this work provides a further understanding of the valleytronics in monolayer transition metal dichalcogenides.