Nonuniversal Transverse Electron Mean Free Path through Few-layer Graphene

Area of Science:

• Condensed matter physics
• Materials science
• Surface science

Background:

• The in-plane electron mean-free path (MFP) in graphene is well-studied.
• The transverse MFP is less understood and often assumed to follow a universal curve.

Purpose of the Study:

• To directly measure transverse electron scattering in graphene.
• To determine quantitative elastic and inelastic MFPs.
• To challenge the assumed universal MFP curve.

Main Methods:

• Direct measurement of transverse electron scattering using low-energy electron microscopy (LEEM) in reflection.
• Transmission electron energy loss spectroscopy (TEELS) for transmission measurements.
• Electron energy range: 0 to 25 eV above the vacuum level.

Main Results:

• The total transverse MFP is only a few graphene layers, even at low energies.
• The elastic MFP oscillates with the number of graphene layers.
• The universal MFP curve is refuted by experimental data.

Conclusions:

• Graphene's transverse electron transport deviates significantly from universal models.
• Theoretical calculations incorporating graphene's band structure support the experimental findings.
• Simple models can reproduce key experimental observations.