Impact of indirect transitions on valley polarization in WS$_2$ and WSe$_2$

TL;DR

This study explains the origin of high valley polarization in few-layer WS$_2$ and its absence in WSe$_2$, highlighting the role of indirect optical transitions and energy minima in the conduction band.

Contribution

It reveals how the relative energy minima at $ m extLambda$- and K-valleys influence valley polarization, advancing understanding for valleytronic device design.

Findings

WS$_2$ exhibits >90% spin-valley polarization at room temperature.

The energy difference between $ m extLambda$- and K-valleys determines polarization.

Temperature and layer number affect valley polarization through energy shifts.

Abstract

Controlling the momentum of carriers in semiconductors, known as valley polarization, is a new resource for optoelectronics and information technologies. Materials exhibiting high polarization are needed for valley-based devices. Few-layer WS$_2$ shows a remarkable spin-valley polarization above 90%, even at room temperature. In stark contrast, polarization is absent for few-layer WSe$_2$ despite the expected material similarities. Here, we explain the origin of valley polarization in both materials based on the interplay between two indirect optical transitions. We show that the relative energy minima at the $\Lambda$- and K-valleys in the conduction band determine the spin-valley polarization of the direct K-K transition. Polarization appears as the energy of the K-valley rises above the $\Lambda$-valley as a function of temperature and number of layers. Our results advance the understanding of the high spin-valley polarization in WS$_2$. This insight will impact the design of both passive and tunable valleytronic devices operating at room temperature.