Spin- and charge-density excitations in quantum dots: a Raman study

In this article we review our recent experimental and theoretical investigations of electronic excitations in GaAs–AlGaAs quantum dots. In the experimental structures we have observed collective spin- (SDE) and charge-density excitations (CDE), and, under conditions of extreme resonance, excitations which can be attributed to nearly unrenormalized single-particle-like transitions (SPE). We compare our experimental results to a theory based on the time-dependent local-density approximation (TDLDA) concerning (i) parity selection rules and (ii) conditions when SPE occur in the spectra. This shows that only a theory which takes into account the resonance enhancements mediated by the valence band states can qualitatively reproduce the experimental findings. To get a deeper understanding of the character of the SPE, we present an intriguing comparison of Kohn–Sham calculations with results of an exact treatment by numerical diagonalization. We demonstrate that even for six electrons per quantum dot, SPE can be expected.