Spin-Orbit Coupling in Graphene and Transition Metal Dichalcogenides
- Author(s): Yang, Bowen;
- Advisor(s): Shi, Jing;
- et al.
Graphene stands out for its high mobility and weak spin-orbit coupling (SOC) offering efficient transport of both electron charges and spins. However, the negligible SOC prevents novel quantum states from emerging, such as the quantum anomalous Hall state. On the other hand, owing to their strong SOC, transition metal dichalcogenides (TMDs) provide an ideal platform to increase the SOC in graphene by proximity effect. This dissertation will focus on the analysis of the induced SOC in graphene, the underlying mechanisms, and the factors that could affect its strength.
The first part briefly introduces graphene and SOC. We present possible ways to increase the SOC in graphene and how to quantify its strength. The second part shows the procedures on how to build a van der Waals heterostructure like graphene/WSe2/h-BN, followed by introductions to Raman and photoluminescence (PL).
In the third part we demonstrate enhanced SOC in monolayer and bilayer graphene on WS2 by magneto-conductance measurements. We will show clear weak antilocalization (WAL) in WS2-covered graphene over a wide range of carrier densities. We isolate and quantify the spin-relaxation rate caused by Rashba SOC and show its strength is tunable via transverse electric fields.
Then we investigate the SOC in graphene coupled to monolayer TMD films. We show that the spin relaxation rate varies linearly with the momentum scattering time and is independent of the carrier type. Our analysis yields a Rashba SOC of ~1.5 meV in graphene/WSe2 and ~0.9 meV in graphene/MoS2. The nearly electron-hole symmetric nature of the Rashba SOC calls deeper understanding for the underlying mechanisms.
Finally, we study the interactions between TMDs and graphene in graphene/WSe2/h-BN and WSe2/ graphene/h-BN. We find that strong PL quenching exists in the former stack while the PL is only weakly quenched in the latter stack. We attribute this difference to the increased interlayer distance between WSe2 and graphene caused by the h-BN, as evidenced by the reduced WAL and the first principles calculations.