Imaging correlated wave functions of few-electron quantum dots:
Theory and STS experiments
Friday, 7 September 2007, 13:30
Seminar room F
Recent experiments have demonstrated that the regime of very few electrons in quantum dots (QDs) displays peculiar properties, different from the many-electron case: Among these, the non-universal behavior in transport interference experiments  and the manifestation of strong correlation effects in light scattering . However, no direct observation of such effects in wave function images was obtained so far. Among the available techniques, scanning tunneling spectroscopy (STS) provides spectacular images of QD wave functions . Experiments so far have shown maps of localized orbitals, that could be explained in terms of an independent-electron model.
In this paper we focus on QDs where we expect that electron-electron interaction may instead be relevant. We show that the wave function actually probed by STS is the space-resolved spectral density amplitude of the one-particle propagator (or quasi-particle wave function), which can considerably deviate from the independent-electron wave function, due to correlation effects . To this aim we investigate, both experimentally and theoretically , STS wave function maps of single and freestanding strain-induced InAs QDs grown on GaAs(001). The sequence of measured wave functions cannot be explained in terms of single-electron orbitals. We compare the measured maps with those predicted by a numerical model which takes into account QD anisotropy and the full correlation effects, and we are able to separately identify ground- and excited-state wave functions corresponding to the injection of a first, second, and perhaps third electron into the QD. This interpretation is supported by the analysis of the measured differential conductance as a function of the stabilization current.
The quasi-particle wave function corresponding to the ground state -> ground state tunneling process N = 1 -> N = 2 displays a surprising two-peak charge modulation which is inconsistent with the simple picture of a doubly occupied s-like orbital. This effect, qualitatively reproduced by our simulations, is due to the destructive interference between different components of the correlated singlet wave function.
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