Spin-orbit interaction (SOI) is an essential building block for novel quantum phenomena such as spin Hall effect or topologically nontrivial states. When applied to two dimensional materials, it can drive graphene into the two-dimensional (2D) topological insulator (quantum spin Hall (QSH) insulator) as first pointed out by Kane and Mele. However, intrinsic SOI in graphene is much smaller than the value assumed in this first theoretical study, and it makes difficult to realize the QSH state in graphene.

In this study, we demonstrate large enhancement of SOI in graphene by means of graphene-transition metal dichalcogenide (TMD) heterostructures. TMDs are also 2D materials similar to graphene, and they have strong intrinsic SOI. In our study, we use MoS2, WSe2 and WS2, as TMDs, and measure magnetoresistance of graphene in proximity to these TMDs. We observe weak-antilocalization behaviors for both samples at low temperatures, a clear signature of induced SOI in graphene. We compare the induced SOI in graphene with different TMDs, and interestingly, we find that induced SOI is stronger with MoS2 than that with WSe2, opposite to the intrinsic amplitude of SOI in these TMDs. We also investigate the symmetric (Kane-Mele type) and asymmetric (Rashba type) contribution to SOI. From the detailed analysis we can draw the conclusion that symmetric SOI is much more dominant in Graphene/MoS2 than the asymmetric one, whereas in Graphene/WSe2 structure these two contributions are almost comparable. Large symmetric SOI induced in graphene with MoS2 is promising to exhibit the QSH state. Our findings on introduction of SOI in graphene reveal that graphene can be a promising material for the QSH state and also spintronics.