Imaging the diffusive-to-ballistic crossover of magnetotransport in graphene

TL;DR

This study uses scanning tunneling potentiometry to visualize how carrier transport in graphene transitions from diffusive to ballistic behavior under varying magnetic fields, revealing detailed local carrier dynamics.

Contribution

It provides nanometer-scale imaging of the diffusive-to-ballistic crossover in graphene's magnetotransport using STP, highlighting the role of carrier motion patterns.

Findings

Transition from diffusive to ballistic transport observed

Distinct potential profile changes at specific radii

Carrier 'spirograph' motion enhances local Hall fields

Abstract

Scanning tunneling potentiometry (STP) is used to probe the local, current-induced electrochemical potential of carriers in graphene near circular electrostatic barriers in an out-of-plane magnetic field ranging from 0 to 1.4 T. These measurements provide nanometer-resolved information about the local motion of carriers, revealing significant changes in carrier dynamics with increasing field strength. At low magnetic fields the electrochemical potential displays a spiral-like pattern, while at high fields it exhibits distinct changes at particular radii. We show that the observed behavior indicates a transition from diffusive to ballistic transport. Additionally, the sharp changes in the measured potential profile at high fields result from the `spirograph' motion of carriers, which creates a local enhancement of the Hall field one cyclotron diameter away from the semiclassical turning point near the electrostatic barrier.