Electronic Elementary Excitations

Abstract

In inelastic light scattering experiments on semiconductor nanostructures, electronic excitations are created or annihilated in the low –dimensional electron systems under investigation. Thus, the main body of this book will deal with the physics of those electronic elementary excitations in various systems and under various conditions. Before we elaborate on the basic concepts of the inelastic light scattering processes themselves in the following chapter, the electronic elementary excitations shall be introduced and discussed here. We will do this by the – most prominent – example of the excitations of Q2D electron systems, realized in modulation–doped GaAs–Al _x Ga_1-x As quantum wells. These excitations can be categorized into so called spin–density excitations (SDE) and charge–density excitations(CDE), which both are collective plasma oscillations of the Q2D system, and, single–particle excitations (SPE). In particular, the observation of intersubband SPE [1] – which are thought to be electronic excitations, which are not affected by the Coulomb interaction – has posed a puzzle, and has been controversially discussed. We will come to this discussion at various places later in this book, when considering the resonant scattering in quantum wells and in quantum dots. In particular in Chap. 5 we will see that – at least for quantum dots – the SPE’s are actually collective excitations: SDE’s and CDE’s. However, the many–particle interaction effects partly cancel under specific conditions so that the energies are close to single–particle energies of a noninteracting system. Historically, in 1979, intersubband CDE and SDE in GaAs–AlGaAs quantum wells [2] and heterojunctions [3] were the first electronic excitations, which were observed in semiconductor nanostructures by inelastic light scattering by A. Pinczuk et al. and G. Abstreiter et al., respectively.