Unusual scarcity in the optical absorption of metallic quantum-dot nanorings described by the extended Hubbard model

We report results of a systematic analysis of optical absorption in finite metallic quantum-dot nanorings containing a variable number of electrons described by the extended Hubbard model. Despite the very strong electron correlations, the number of significant spectral lines is astonishingly small, and the optical spectra can still be rationalized within a simple framework relying upon the single-particle picture. The main effect of correlations is to split the optical transitions that are degenerate within the single-particle description and to give rise to numerous avoided crossings (anticrossings). Unusually, the latter often involve more than two states. For closed-shell systems, the optical spectrum is practically monochromatic. We also present results of an approximate scheme, based on the Landau idea to describe low excitations in an interacting electron system. It consists of diagonalizing the Hamiltonian in the configuration interaction truncated to include dressed particle-hole excitations derived from the exact ground state. This method provides a good description of the overall optical absorption, including that of the diabatic state corresponding to the bright transition at avoided crossings. The scarcity in the optical spectra reported here points toward a hidden dynamical quasisymmetry in the optical absorption of finite nanorings described within the extended Hubbard model, generalizing thereby our recent finding [I. Baldea and L. S. Cederbaum, Phys. Rev. B 75, 125323 (2007)]. In addition, we report a qualitative difference between the optical gap and the charge gap occurring at quarter filling.