Regensburg 2010 – scientific programme
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O: Fachverband Oberflächenphysik
O 82: Graphene IV
O 82.7: Talk
Friday, March 26, 2010, 12:45–13:00, H31
Plasmons in epitaxial graphene: influence of steps and doping concentration — •Thomas Langer1,2, Herbert Pfnür1, Christoph Tegenkamp1, and Hans Werner Schumacher2 — 1Institut für Festkörperphysik, Leibniz Universität Hannover, D-30167 Hannover — 2PTB, D-38116 Braunschweig
Collective excitations of the two dimensional electron gas in epitaxial graphene grown on SiC(0001) have been studied. Structural properties were analyzed by XPS and SPA-LEED. The influence of surface defects on the dispersion and the lifetime of the sheet plasmon has been investigated in detail by angle resolved EELS. The step concentration (10-100nm terrace length) has been varied by using differently ex-situ treated samples and by varying the sublimation rates. The dispersion, which does not depend on the defect concentration, is almost linear and differs strongly from a √k behaviour, which is often found for 2DEG. The strong blueshift in the short wavelength regime can be attributed to nonlocal effects within the highly correlated electron system. For the pristine graphene layer the dispersion shows a pronounced dip at the position of kF, which vanishes after adsorption of 4F-TCNQ. Hence the position can be used to control the filling of the conduction band. The lifetime of the plasmons is inversely proportional to the step density and to the wavelength. As the phase velocities of the plasmons deduced from the linear dispersion coincide with the Fermi velocity of the electrons, steps only have to act as a momentum source to couple effectively plasmons to the electron-hole continuum, thus reducing effectivly the lifetime outside the Landau regime.