Evidence for Quasi Two-Dimensional Positronium Formation in Potassium-Intercalated Graphite

Two-dimensional angular correlation of annihilation radiation (2D ACAR) experiments have been performed to study the electron-positron momentum densities in potassium-intercalated graphite and in highly oriented pyrolytic graphite. Magnetic-field-dependent experiments indicate that the high-intensity, anisotropic 2D ACAR component observed in ${\mathrm{C}}_{24}$K is at least partially due to quasi-2D positronium formation. In the highly oriented pyrolytic graphite 2D ACAR spectra, a low-intensity, nearly isotropic, thermally activated positronium component is found.