Magnetic field induced polarization enhancement in the photoluminescence of MBE-grown WSe$_2$ layers

TL;DR

This study demonstrates that a weak magnetic field significantly enhances valley polarization in MBE-grown WSe$_2$ monolayers, revealing differences from exfoliated samples and providing insights into exciton dynamics.

Contribution

It presents the first experimental investigation of magnetic-field effects on valley polarization in MBE-grown WSe$_2$ monolayers, highlighting differences from exfoliated samples.

Findings

Weak magnetic field increases valley polarization effectiveness.

Time-resolved PL shows a pseudospin relaxation time of ~25 ps.

Differences observed between MBE-grown and exfoliated WSe$_2$ samples.

Abstract

We report an experimental study of the magnetic-field dependence of the optically pumped valley polarization in an epitaxial tungsten diselenide (WSe$_2$) monolayer grown by molecular-beam epitaxy (MBE) on a hexagonal boron nitride (hBN) substrate. Circularly polarized photoluminescence (PL) measurements reveal that applying a weak out-of-plane magnetic field, on the order of 0.1 T, dramatically increases the effectiveness of the optical orientation of the emission associated with defect-bound localized excitons. We compare the obtained results with the earlier studies on the reference exfoliated monolayers, discussing both qualitative similarity as well as quantitative differences. Our observations are further supplemented by the results of time-resolved PL measurements, which confirm the pseudospin relaxation time of approximately 25 ps, a value significantly shorter than the $\approx$100 ps previously reported for mechanically exfoliated samples.