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We calculate the effective charge for multimagnon infrared (IR) absorptionassisted by phonons in a perovskite like antiferromagnet and we compute thespectra for two magnon absorption using interacting spin-wave theory. The fullset of equations for the interacting two magnon problem is presented in therandom phase approximation for arbitrary total momentum of the magnon pair. Thespin wave theory results fit very well the primary peak of recent measuredbands in the parent insulating compounds of cuprate superconductors. The lineshape is explained as being due to the absorption of one phonon plus a newquasiparticle excitation of the Heisenberg Hamiltonian that consists off a longlived virtual bound state of two magnons (bimagnon). The bimagnon states havewell defined energy and momentum in a substantial portion of the Brillouinzone. The higher energy bands are explained as one phonon plus highermultimagnon absorption processes. Other possible experiments for observingbimagnons are proposed. In addition we predict the line shape for the spin onesystem La$_2$NiO$_4$.

Theory of phonon-assisted multimagnon optical absorption and bimagnon states in quantum antiferromagnets