Spectroscopy of nanoscopic semiconductor rings

Making use of self-assembly techniques, we realize nanoscopic semiconductor quantum rings in which the electronic states are in the true quantum limit . We employ two complementary spectroscopic techniques to investigate both the ground states and the excitations of these rings. Applying a magnetic f ield perpendicular to the plane of the rings, we find that, when approximat ely one flux quantum threads the interior of each ring, a change in the gro und state from angular momentum l = 0 to l = -1 takes place. This ground st ate transition is revealed both by a drastic modification of the excitation spectrum and by a change in the magnetic-field dispersion of the single-el ectron charging energy.