Coupled Plasmons and Single Particle Excitations in the Two-Dimensional Electron Gas

Abstract

The motion, the excitations and the dielectric response of carriers parallel to the two-dimensional electron layers of modulation doped heterojunctions and multi-quantum wells are characterised by collective excitations (density oscillations = plasmons) and single particle excitations (acceleration of single electrons). These excitations correspond to spin density and charge density fluctuations. They determine the response of layered electron systems to electromagnetic fields, such as screening of the Coulomb interaction, e.g., of impurities, or the conduction processes. We show experimentally, that a layered electron system with N layers has N discrete plasmon eigenmodes. Measuring these modes provides a method to study a range of interactions in the system: the Coulomb interaction between layers, and interaction of the plasmons with other excitations. The discrete layer plasmon modes fan out into a band due to the Coulomb interaction between electrons in different layers. We have measured the dispersion of the discrete layer plasmon eigenmodes as a function of the in-plane wave vector k|| for modulation doped GaAs/AlGaAs quantum wells with a small number of layers (N = 2... 20). These measurements allow to test the large number of theoretical calculations of the plasmon dispersion in layered systems. They also provide a technique to characterise electron densities in layered electron gas systems. Thus measurement of the 3D plasmon frequency is currently used to determine the carrier concentration in doped bulk semiconductors. We have measured the wavevector and temperature dependence of the single particle excitation (SPE) spectra. The SPE spectra can be well fitted by a Lindhard-Mermin dielectric response function, and we determine the temperature dependence of the single particle relaxation time for our sample.