Direct imaging of magnetotransport at graphene-metal interfaces with a single-spin quantum sensor

Magnetotransport underlines many important phenomena in condensed matter physics, such as the Hall effect and magnetoresistance (MR) effect, and forms the basis for applications in magnetic memories and spintronic devices. Thus far, most magnetotransport studies have been based on bulk resistance measurements, without direct access to the nanoscale spatial transport pattern. In this talk, we discuss nanoscale quantum imaging of magnetotransport in a monolayer graphene with an embedded metal disc using a scanning nitrogen-vacancy magnetometer. By visualizing the current flow around charge-neutrality under an out-of-plane magnetic field of ~0.5 T, we directly observe Lorentz-force-induced current deflection at the graphene-to-metal interface with a low contact resistance. As the carrier density in graphene increases, the current increasingly flows through the graphene sheet. In addition, we observe that the MR is more prominent in the ambipolar regime compared to the electron- or hole- doped regimes, which can be attributed to the intrinsic MR effect. Finally, we show that spatial current imaging uncovers non-uniform contact resistances along the circular graphene-metal interfaces, which cannot be easily identified by optical, electrical or topographic characterization.