Electrical transport study of Graphene/WSe2 Heterostructure

Abstract

Spintronics has emerged as a novel platform to build information storage and logic devices in the last few decades. Graphene promises a long spin propagation length even at room temperature due to its low spin-orbit interaction strength and high mobility. However, spin manipulation is tricky in graphene because of its low spin-orbit interaction (SOI) strength. To achieve the ability to manipulate spins in the graphene channel, we fabricated the graphene's van der Waals (vdW) heterostructures with WSe2. Due to the proximity effect, a significant SOI is induced in the graphene channel, while the high mobility ensures a considerable spin propagation length. Bilayer graphene (BLG), due to its layer degree of freedom, offers more tunability than its monolayer counterpart. The application of the perpendicular displacement field can be used to change the polarizability of each layer and hence the low energy bands. In the BLG/ WSe2 heterostructures, the layer close to WSe2 will experience a higher proximity effect than the other layer. We utilized this property to realize the theoretically predicted spin-orbit valve in BLG. We found the transition from weak antilocalization to weak localization as the sign of the displacement field is changed from negative to positive while maintaining the Fermi energy in the valence band [1]. This observation supports the transfer of strength of SOI from one band to another band. The tunability of the SOI by a displacement field is further supported by the high field Shubnikov-de Haas oscillations (SdH) measurements. Furthermore, we report the experimental observation of topological edge-states in hBN/BLG/WSe2/hBN heterostructures when the bulk bands are gapped out at large displacement fields. We find a pair of topological time-reversal symmetric helical edge modes at the edge of the sample, which is characterized by the quantization of the electrical resistance in multiterminal geometries [2]. The observed values of the quantized electrical resistance are in accord with the Landauer-Buttiker formalism for helical edge modes. [1] P. Tiwari et al. Electric-field-tunable valley Zeeman effect in bilayer graphene heterostructures: Realizing the spin-orbit valve effect. Phys. Rev. Letts. 126 096801 (2021).

[2] P. Tiwari et al. Observation of Time-Reversal Invariant Helical Edge-Modes in Bilayer Graphene/WSe2 Heterostructure. ACS Nano 15 916, (2021)