Two-magnon excitations in cuprates and nickelates

We discuss two-magnon phonon-assisted infrared absorption in two-dimensional Heisenberg systems like the parent insulating high-T-c cuprates (spin 1/2) and nickelates (spin 1). The theoretical line shape is compared with experiments. In the case of the cuprates it explains the mid-infrared peaks observed in the insulator. In the case of the nickelates the predicted line shape is also shown to agree with the experiments. We discuss the possibility to observe these excitations in other experiments.