Evidence of Electron Correlations in Plasmon Dispersions of Ultra-Low Density 2D Electron Systems

Ultra-low density 2D electron systems are probed by inelastic light scattering at wavevectors large enough for both correlation and non-local effects to be significant. We find well-defined plasmons with dispersions that deviate from the "classical" square-root of q limit. At lower temperatures, the deviation is negative and scales with the interparticle spacing and becomes positive as temperature increases. These results are interpreted as evidence of large correlation effects, arising from the predominance of electron interactions over non-local corrections at low temperatures.