Collective Excitations in Low Density 2D Electron Systems

We report the observation of sharp plasmon and magnetoplasmon modes in ultra-low density 2D electron systems. Well-defined dispersions for the modes are observed at densitities as low as 1.1 x 10 sup 9 cm sup (-2) and with excitation wave vectors as large as 1.2 x 10 sup 5 cm sup (-1). Interestingly, both modes are found to be more easily measured in low density systems than in high density systems. The strength of the light scattering cross-sections at low density suggests potential applications to the study of quantum phase transitions at large r sub s.