Phonon-assisted IR spectroscopy of quantum antiferromagnets

We review recent theoretical results for multimagnon-phonon assisted infrared absorption in antiferromagnetic Heisenberg systems. We show spin wave theory line shapes for 2D spin 1/2 systems (like the parent insulating high-Tc cuprates) 1D spin 1/2 systems and 2D spin 1 systems (like the nickelates) and exact diagonalization results in two-dimensional spin 1/2 systems. The theoretical line shapes are compared with experiments. In the case of cuprates they explain mid-infrared peaks observed in the insulator. In the case of the nickelates a predicted line shape is also shown to agree with the experiments. We discuss the possibility to observe these excitations in other experiments.