NANO-CORR · Signatures of few-body correlations in semiconductor quantum nanostructures

Final Activity Report Summary - NANO-CORR (Signatures of few-body correlations in semiconductor quantum nanostructures)

Semiconductor quantum dots (QDs) are man-made nanostructures which can be charged with a controllable number of electrons. Their electronic energy spectrum is discrete, as in natural atoms. QDs' mimicking natural atoms -but with improved flexibility in the design- is of great interest for technological applications. However, their strong interaction with the environment, mainly via vibrations of the semiconductor lattice -phonons- is a major handicap, since it often makes the lifetime of the electronic excitations too short for practical purposes -few ns for typical materials as GaAs. Our results, however, show that this value can be greatly enhanced by charging the QDs with more than one electron, since Coulomb correlations tend to reduce the electron-phonon interaction. As opposed to conventional electronics (exploiting the charge of electrons), recent technological developments make it possible to exploit the spin of electrons -a new technological paradigm known as spintronics. In QDs the spin lifetime can be as large as several ms, which is clearly more favourable than charge excitations. In this context, we have reported the first study of the spin excitation lifetime in QDs with interacting confined electrons and magnetic fields. Our results showed that the lifetime can be greatly modulated by the magnetic field, due to the changes in the inner electronic structure, this prediction having been accurately confirmed by later experiments at the University of Delft. Furthermore, we have shown that few-electron states may display longer spin lifetime than one or two-electron ones. When two QDs are placed close to each other, they couple much as natural atoms form molecules, thus forming 'artificial molecules'. These structures are currently under intensive research due to their potential applications in Quantum Information Processing. To this end, the dynamics of electrons oscillating between the two QDs constituting an artificial molecule is a basic and interesting problem that needs to be understood. When only one electron is trapped in the molecule, the oscillation process is clear from elementary quantum mechanics: the electron moves back and forth with a simple period. However, when more than one electron is present, Coulomb interaction and correlations complicate the process. We have elucidated how the number of interacting electron affects these oscillations and shown that the strong correlations can explain the surprisingly simple oscillation patterns observed in experiments with many electrons. QDs can be charged both with conduction electrons and valence holes. In many respects, holes can be thought of as electrons with positive charge and heavier mass. Yet, holes have the particularity of being subject to strong spin-orbit interaction. We have predicted [6] an unexpected effect due such interaction in artificial molecules charged with holes, namely the formation of a ground state with antibonding character, a situation which never occurs in natural molecules where the ground state is always bonding, nor in artificial molecules charged with conduction electrons which follow the standard rules of quantum mechanics. Novel properties never observed in ordinary materials may be expected from this exotic molecular state, whose existence was lately confirmed experimentally at NRL (Washington). Finally, we have shown that -owing to their heavy mass- the behaviour of holes is much more than electrons by Coulomb correlations. By properly accounting for correlations, we have explained recent experimental observations in self-assembled QDs -the most promising kind of dot for integrated optoelectronic devices-.

Текст от CORDIS, на английски · Данни: CORDIS, © Европейски съюз