Analysis of Scanned Probe Images for Magnetic Focusing in Graphene

TL;DR

This paper demonstrates how a cooled scanning probe microscope can image and analyze electron trajectories in ballistic graphene devices, revealing effects of tip-induced charge density and edge roughness on electron flow.

Contribution

It introduces a novel application of SPM for imaging magnetic focusing in graphene and analyzes the impact of tip-induced charge on electron trajectories.

Findings

Tip-induced charge density dip affects electron paths.

Graphene edges are rough, causing random scattering.

Tip proximity can enhance transmission by redirecting electrons.

Abstract

We use a cooled Scanning Probe Microscope (SPM) to electron motion in nanoscale devices. The charged tip of the SPM is raster scanned at a constant height above the surface as the conductance of the device is measured. The image charge scatters electrons away, changing the path of electrons through the sample.1-3 Using this technique, we have imaged cyclotron orbits3 for ballistic hBN-graphene-hBN devices that flow between two narrow contacts in the magnetic focusing regime. Here we present an analysis of our magnetic focusing imaging results based on the effects of the tip-created charge density dip on the motion of ballistic electrons. The density dip locally reduces the Fermi energy, creating a force that pushes electrons away from the tip. When the tip is above the cyclotron orbit, electrons are deflected away from the receiving contact, creating an image by reducing the transmission between contacts. The data and our analysis suggest that graphene edge is rather rough, and electrons scattering off the edge bounce in random directions. However, when the tip is close to the edge it can enhance transmission by bouncing electrons away from the edge, toward the receiving contact. Our results demonstrate that a cooled SPM is a promising tool to investigate the motion of electrons in ballistic graphene devices.