EXACT DIAGONALIZATION RESULTS FOR MULTIMAGNON IR ABSORPTION IN THE CUPRATES

Recent measured bands in the mid IR of parent insulating compounds of cuprate superconductors [Perkins et al. Phys. Rev. Lett. 71 1621 (1993 )] are interpreted as multimagnon infrared (IR) absorption assisted by phonons. We present results for the coupling constant of light with t his excitations and exact diagonalization results for the spectra and compare with interacting spin wave theory. The observed structures cor respond to the absorption of one phonon plus two magnons and one phono n plus higher multimagnon absorption processes. Our results imply that IR absorption can be used as a unique probe of quantum antiferromagne tic systems.