Imaging charge carrier dynamics and phase transitions using scanning probe potentiometry

I will discuss how scanning tunneling potentiometry (STP) can be used to measure the electrochemical profile of current-biased graphene as the quasiparticles transition through different electronic phases. I will first describe how STP maps relate to local current density distribution of a sample. I will then demonstrate how the scanned probe tip can be used to draw electrostatic barriers in the device which act to locally perturb the current flowing through the graphene sheet. Subsequent STP measurements of the current profile around circular barriers and through constrictions reveal that at low temperatures the flow is ballistic, but as the temperature is raised it becomes viscous due to a phase transition of the charge carriers in the graphene. In the viscous regime, interesting features in the flow can be observed including greater-than-ballistic conductance through narrow channels, and Landaur residual resistivity dipoles with reduced magnitude. Measurements with applied magnetic fields, meanwhile, show clear changes in carrier trajectories that can be related to cyclotron orbits in the diffusive and quantum hall regimes.