[Jlab-seminars] [Jlabur_clqxpl] [Ur_colloq] COLLOQUIUM ANNOUNCEMENT

[Luci Collins]

*Colloquium
Wed., 11/17/10 at 4:00PM
CEBAF Center AUD.
Cookies & Coffee at 3:45PM

**"The Effective Fine Structure Constant of Graphene."**
*Peter Abbamonte
University of Illinois, Urbana-Champaign.*

The Nobel Prize in physics this year was awarded to Andre Geim and 
Konstantin Novoselovfor work on Graphene - Carbon's New Face.

Graphene is a thin flake of ordinary carbon, just one atom thick, in 
such a flat form that it has exceptional properties that originate from 
the remarkable world of quantum physics. As a material it is completely 
new -- not only the thinnest ever but also the strongest. As a conductor 
of electricity it performs as well as copper. As a conductor of heat it 
outperforms all other known materials. It is almost completely 
transparent, yet so dense that not even helium, the smallest gas atom, 
can pass through it. Carbon, the basis of all known life on earth, has 
surprised us once again.

Electrons in graphene behave like Dirac fermions, permitting phenomena 
familiar from high energy nuclear physics to be studied in a solid state 
setting. A key question is whether or not these Fermions are critically 
influenced by Coulomb correlations. In this talk I will describe 
inelastic x-ray scattering experiments on crystals of graphite, which we 
have used, in conjunction with new reconstruction algorithms, to image 
the dynamical screening of charge in a freestanding, graphene sheet.  We 
found that the polarizability of the Dirac fermions is enhanced by 
excitonic effects, improving the screening of interactions between 
renormalized quasiparticles.  The strength of interactions is 
characterized by a scale-dependent, effective fine structure constant, 
alpha(k,omega), whose value approaches ~1/7 at low energy and large 
distances.  I will discuss the implications of this result for various 
phenomena, such as the screening of charged impurities, and will outline 
more generally how time-resolved x-ray techniques can make an impact in 
condensed matter physics.
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