Polarization and Aharonov-Bohm oscillations in quantum-ring magnetoexcitons

We study interaction and radial polarization effects on the absorption spectrum of neutral bound magnetoex- citons confined in quantum-ring structures. We show that the size and orientation of the exciton's dipole moment, as well as the interaction screening, play important roles in the Aharonov-Bohm AB oscillations. In particular, the excitonic absorption peaks display AB oscillations both in position and amplitude for weak electron-hole interaction and large radial polarization. The presence of impurity scattering induces anticross- ings in the exciton spectrum, leading to a modulation in the absorption strength. These properties could be used in experimental investigations of the effect in semiconductor quantum-ring structures. In this paper, we show that the optical absorption in semi- conductor quantum rings is governed by the interplay be- tween Coulomb interactions and the excitonic radial polar- ization. The model for the polarized quantum-ring magnetoexcitons is presented in Sec. II and our main results are shown in Sec. III. Ground-state AB oscillations are prominent when interactions are weak due to, e.g., strong screening by a metallic gate, and the oscillatory pattern changes when the electric dipole vector is reversed, as shown in Sec. III A. In the fully interacting weak screening regime Sec. III B, the oscillations are suppressed in the lowest op- tically active state due to the "Coulomb locking" of the elec- tron and the hole. Moreover, as the magnetic field increases, the ground-state changes its angular momentum from 0 to 1, becoming optically inactive, and corroborating an earlier qualitative analysis of the strongly interacting limit. 9 The op- tically active excited states are shown to display a rich struc- ture of AB oscillations and field-dependent absorption modu- lations. Most importantly, we show that the gap between the ground and the excited states can be tuned by changing the exciton's electric dipole moment, allowing for an experimen- tal characterization of the excited states of quantum-ring structures. Furthermore, in Sec. III C we analyze how the scattering due to impurities along the ring affects the optical absorp- tion. The impurity-induced coupling between exciton states leads to a modulation in the absorption as a function of mag- netic field, present even for very weak scattering potential strengths.