Zachary Krebs

Imaging viscous and nonlocal transport of electron fluids with scanning tunneling potentiometry

Abstract: Ballistic and hydrodynamic electron flow can develop in materials when carrier momentum is conserved over long distance and time scales. These non-Ohmic transport regimes are characterized by distinctive spatial distributions of the current density and electrochemical potential. In this talk, I will show scanning tunneling potentiometry (STP) measurements of the electrochemical potential induced by DC transport in graphene as a function of carrier density, temperature, and magnetic field. First, STP images are recorded as current flows through tunable electrostatic constrictions that are "drawn" with the STM tip. Upon heating the system from 4.5 K to 77 K, enhanced electron-electron scattering leads to a crossover from ballistic to hydrodynamic flow, identified by superballistic conductance through the constrictions and a suppression of Landauer residual resistivity dipoles. When increasing the magnetic field from 0 to 1.4 T at 4.5 K, the STP data reveals a diffusive-to-ballistic crossover in the flow of current resulting from Landau level quantization. In the ballistic regime of magnetotransport, the local Hall field is enhanced one cyclotron diameter away from scattering surfaces. Bio: Zach Krebs is a graduate researcher in Prof. Victor Brar's group at the University of Wisconsin - Madison. During his Ph.D., Zach used a variety of cryogenic scanned probe techniques to image the local electronic structure and charge carrier dynamics of two-dimensional metals and semiconductors. He earned his B.S. in Physics from the University of Chicago in 2017, where he worked with Prof. Stuart Rice to understand transient fluctuations in the hexatic phase of colloidal systems. Currently, Zach is interested in using local probes to further investigate viscous flow in electron fluids.