Generic Optical Excitations of Correlated Systems: π -tons

The interaction of light with solids gives rise to new bosonic quasiparticles, with the exciton being—undoubtedly—the most famous of these polaritons. While excitons are the generic polaritons of semiconductors, we show that for strongly correlated systems another polariton is prevalent—originating from the dominant antiferromagnetic or charge density wave fluctuations in these systems. As these are usually associated with a wave vector (π ,π ,…) or close to it, we propose to call the derived polaritons π -tons. These π -tons yield the leading vertex correction to the optical conductivity in all correlated models studied: the Hubbard, the extended Hubbard model, the Falicov-Kimball, and the Pariser-Parr-Pople model, both in the insulating and in the metallic phase.