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 (pi, pi, ...) or close to it, we propose to call the derived polaritons pi-tons. These pi-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.