Unusual Features in Optical Absorption and Photo-Ionisation of Quantum Dot Nano-Rings

Abstract

We present theoretical results on nano-rings consisting of silver quantum dots (QD’s) described within the extended Hubbard model. The parameter values, taken from literature, can be tuned in a controlled way in wide ranges, a fact that makes QD assemblies particularly interesting. As a result, such nano-systems can be smoothly driven from a weak correlation to a strong correlation regime. To reveal the cross-over between these two regimes, we present results on optical absorption and photo-ionisation. Astonishingly, although the picture based on molecular orbitals (MO’s) completely breaks down in the strong correlation regime, both optical absorption and ionisation spectra are surprisingly simple, and can be rationalised within the MO picture. Therefore, the information obtained in this way is scarce, and we show that, by partially covering the nano-rings, is can be considerably enriched. In the nano-rings investigated here we observe the phenomenon called avoiding crossing by molecular physicists and anti-crossing by the solid state community. Unlike in all other cases of avoiding crossings of which we are aware, we often encounter situations where more than two states of identical symmetry are involved.