Spin and Valley Polarization Dynamics Probed by Time-Resolved Kerr Rotation in WSe₂

Using time-resolved Kerr rotation measurements, we observe long-lived spin signals up to ~80 ns in the MOCVD-grown monolayer transition metal dichalcogenide (TMD) WSe₂ [1]. This signal is robust to transverse magnetic fields, providing evidence that the spin signals arise from the resident hole population. A recent study by Dey et al. has shown this robustness occurs solely in the p-type regime, due to spin-valley locking in the valence band [2]. Photoluminescence measurements also show a transition from free exciton to localized exciton emission as the temperature decreases. Wavelength-dependent Kerr rotation measurements then reveal that this polarization is only generated by excitation near the free exciton energy. Unlike a recent study on exfoliated MoSe₂ which shows long spin lifetimes at room temperature, our polarization decays quickly as the temperature is increased [3]. Combined, these measurements support the future use of TMDs for creating spintronic and valleytronic devices.
[1] X. Song et al., Nano Lett. 16, 5010 (2016)
[2] P. Dey et al., Phys. Rev. Lett. 199, 7401 (2017)
[3] M. Ersfeld, et al., arXiv:1708.00228v1