Plasmon Dispersion and Damping at the Surface of a Semimetal

We have investigated for the first time the dispersion and the momentum dependence of the damping of the surface plasmon of a semimetal, the low-energy $\pi$ band plasmon of graphite, using high resolution electron energy loss spectroscopy. The measured dispersion is positive. In contrast to all previous studies on metals, the width of the plasmon loss decreases substantially with increasing parallel momentum and appears to collapse to a residual value beyond a critical momentum. These findings are explained in terms of the coupling to interband transitions in semimetallic graphite.