THEORY OF PHONON-ASSISTED MULTIMAGNON OPTICAL-ABSORPTION AND BIMAGNONSTATES IN QUANTUM ANTIFERROMAGNETS

We calculate the effective charge for multimagnon infrared absorption assisted by phonons in a perovskitelike antiferromagnet and we compute the spectra for two-magnon absorption using interacting spin-wave the ory. The full set of equations for the interacting two-magnon problem is presented in the random-phase approximation for arbitrary total mom entum of the magnon pair. The spin-wave theory results fit very well t he primary peak of recent measured bands in the parent insulating comp ounds of cuprate superconductors. The line shape is explained as being due to the absorption of one phonon plus a new quasiparticle excitati on of the Heisenberg Hamiltonian that consists of a long-lived virtual bound state of two magnons (bimagnon). The bimagnon states have well- defined energy and momentum in a substantial portion of the Brillouin zone. The higher-energy bands are explained as one phonon plus higher multimagnon absorption processes. Other possible experiments for obser ving bimagnons are proposed. In addition we predict the line shape for the spin-1 system La2NiO4.