2-MAGNON EXCITATIONS IN CUPRATES AND NICKELATES

We discuss two-magnon phonon-assisted infrared absorption in two-dimen sional Heisenberg systems like the parent insulating high-T-c cuprates (spin 1/2) and nickelates (spin 1). The theoretical line shape is com pared with experiments. In the case of the cuprates it explains the mi d-infrared peaks observed in the insulator. In the case of the nickela tes the predicted line shape is also shown to agree with the experimen ts. We discuss the possibility to observe these excitations in other e xperiments.