Raman measurements are reported of collective modes (plasmons) and single-particle excitations in modulation-doped $\mathrm{Ga}\frac{\mathrm{As}}{{\mathrm{Al}}_x}{\mathrm{Ga}}_{1-x}\mathrm{As}$ quantum-well systems as a function of the wave-vector component $k_{\parallel }$ parallel to the electron layers. Samples with five layers show discrete layer-plasmon modes manifesting the coupling of the electrons in different layers by Coulomb interaction. The Raman spectra due to single-particle excitations are narrow bands peaking at $\overline{h}{k}_{\parallel }{v}_{\mathrm{F}}^{2\mathrm{D}}$ characteristic for the response of a 2D electron gas.

• Received 18 September 1985

DOI:https://doi.org/10.1103/PhysRevLett.56.2517