Evidence of electron correlations in plasmon dispersions of ultralow density two-dimensional electron systems

Ultralow density two-dimensional electron systems are probed by inelastic light scattering at wave vectors large enough for both correlation and nonlocal effects to be significant. We find well-defined plasmons with dispersions that deviate from the ``classical{''} rootq 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 nonlocal corrections at low temperatures.