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<div align="left"><font color="#cc0000"><big><b>Colloquium <br>
Wed., 11/17/10 at 4:00PM<br>
CEBAF Center AUD.<br>
Cookies & Coffee at 3:45PM<br>
<font color="#000099"><br>
</font></b></big></font><big><b><font color="#000099">"The Effective
Fine Structure Constant of Graphene."*</font></b><br>
<font color="#000099"><b>Peter Abbamonte<br>
University of Illinois, Urbana-Champaign.</b>
<br>
<br>
</font></big>The Nobel Prize in physics this year was awarded to Andre
Geim and Konstantin Novoselovfor work on Graphene - Carbon's New Face.
<br>
<br>
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.<br>
<br>
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.
<br>
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