TEMPERATURE-DEPENDENT PLASMON FREQUENCY AND LINEWIDTH IN A SEMIMETAL

High-resolution electron-energy-loss spectroscopy of the surface of graphite reveals a discrete loss feature attributed to the excitation of a pi-band plasmon polarized with E parallel-to c. The frequency of the plasmon is found to be strongly temperature dependent. This arises from the contribution of thermally generated carriers to the plasma frequency, as confirmed by explicit calculations using the graphite band structure. From the observed temperature dependence of the plasmon width, we also obtain quantitative details of the plasmon decay channel, which is found to be dominated by the low-frequency electron-hole pair excitations characteristic of a semimetal. Similar effects are expected in other semimetals.