Experimental Model of Graphene

Interlayer Interaction and Electronic Screening in Multilayer Graphene Investigated with Angle-Resolved Photoemission Spectroscopy

T. Ohta, et al.

PRL 98, 206802 (2007)

URL: http://link.aps.org/abstract/PRL/v98/e206802

This group from Berkeley has used the Advanced Light Source to generate a phenomenological tight-binding model for graphite samples with 1 to 4 layers. They used angle-resolved photoemission to construct the energy bands of their samples, then tuned the parameters of their tight-binding model to reproduce the observed band structure.

There were several interesting findings:

1) The splitting of the pi-electron bands in a 4-layer sample is nearly identical to that in bulk graphite. However, the hopping integral is larger in the layered sample and the screening length is smaller.

2) The charge density in the samples was nearly constant. It did not depend on the number of layers or the way in which they were stacked.

3) In layered samples, the oscillations of photoemission intensity as a function of momentum perpendicular to the beam are similar to those observed in quantized thin film states.

4) Single graphene sheets show no oscillations of this nature. The authors point out that there is more to this than the fact that graphene is only a single layer of carbon atoms. It indicates that there is virtually no interaction with the substrate. Graphene really is like an isolated 2D system.

5) The on-site Coulomb interactions differ from layer to layer. I have not seen this effect included in simple models of graphene bilayers. Then again, it's been a while since I read one of those papers.