Quantum transport studies on graphene-transition metal dichalcogenides heterostructures

Abstract

Spin-orbit coupling (SOC) provides unique capabilities to control spin by moving electrons around or turn electron trajectories by rotating spin. On the other hand, van der Waals engineering enables us to combine different electronic properties of two-dimensional (2D) quantum materials through creative assembly. In particular, graphene-transition metal dichalcogenide (TMDC) heterostructure has been a system of interest since one can induce strong SOC in graphene while simultaneously preserving its large experimental flexibility.

This thesis mainly presents our quantum transport studies on monolayer and bilayer graphene-WSe2 heterostructures. By measuring the ballistic transverse magnetic focusing (TMF), we have successfully demonstrated ballistic transport and spectroscopic evidence of the spin-orbit-coupled bands, as well as probed electron dynamics that are sensitive to the edge scattering and temperature. Further, the difference in TMF and Shubnikov–de Haas (SdH) oscillation measurements indicates a fundamental origin behind it, calling for in-depth studies. Our study illustrates, unambiguously, that it is possible to exploit a variety of highly-tunable ballistic transport effects in graphene, such as TMF, Veselago lensing, Fabry-Pérot interference, and more, to control or detect spin by turning ballistic electron motion. This adds graphene van der Waals heterostructures as one of the potential platforms in the field of ballistic spin-orbitronics.

In addition, we have also developed a microneedle manipulation technique, where one can scratch the flakes into a designated shape with a precision at micrometer scales, move, rotate, roll-up, and exfoliate the flakes to help building various types of heterostructures, and form electric contacts by directly drawing/placing thin metal wires over the flake. It therefore can help us to fabricate devices from different 2D materials irrespective of their sensitivities to the environments.