Imaging quasiparticle flow around electrostatic barriers in graphene using scanning tunneling potentiometry and Kelvin probe force microscopy

I will discuss how scanning tunneling potentiometry (STP) and Kelvin probe force microscopy (KPFM) can be used to quantify and image the electrochemical profile of graphene/hBN devices biased with in-plane currents. It will be shown how these measurements can reveal the profiles of in-plane electrostatic potentials, and to probe the local current density in the graphene sheet. By using an STM tip to draw electrostatic barriers, the flow profile of the quasiparticle current is investigated as it passes around circular barriers and through constrictions of different characteristic widths. I will show that for ballistic transport, the wavefunctions of carrier states can be directly imaged, revealing scattering states with novel properties. By increasing the sample temperature from 4.5K to 77K, it will be shown that the flow profile evolves in a manner that is consistent with a Knudsen-to-Gurzhi regime crossover, as the quasiparticles transition to a hydrodynamic phase. In particular, the conductivity of local constrictions is observed to exceed the ballistic limit, and localized charge accumulation/depletion is observed on the upstream/downstream side of barriers.