Single layer transition metal dichalcogenides are 2D semiconducting systems with unique electronic band structure. Two-valley energy bands along with strong spin-orbital coupling lead to valley-dependent carrier spin polarization, which is the basis for recently proposed valleytronic applications. These systems also exhibit unusually strong many body effects, such as strong exciton and trion binding, due to reduced dielectric screening of Coulomb interactions. Not much is known about the impact of strong many particle correlations on spin and valley polarization dynamics. Here we report direct measurements of ultrafast valley specific relaxation dynamics in single layer MoS₂ and WS₂. We found that excitonic many body interactions significantly contribute to the relaxation process. Biexciton formation reveals hole valley/spin relaxation time in MoS₂. Our results suggest that initial fast intervalley electron scattering and electron spin relaxation leads to loss of valley polarization for holes through an electron-hole spin exchange mechanism in both MoS₂ and WS₂.

Date Published: 5 September 2014
PDF: 10 pages
Proc. SPIE 9198, Ultrafast Nonlinear Imaging and Spectroscopy II, 91980K (5 September 2014); doi: 10.1117/12.2062401

Published in SPIE Proceedings Vol. 9198:
Ultrafast Nonlinear Imaging and Spectroscopy II
Zhiwen Liu, Editor(s)