J. Lorenzana et al., Does the Heisenberg model describe the multimagnon spin dynamics in antiferromagnetic CuO layers?, PHYS REV L, 83(24), 1999, pp. 5122-5125
We compute the absorption spectrum for multimagnon excitations assisted by
phonons in insulating layered cuprates using exact diagonalization in clust
ers of up to 32 sites. The resulting line shape is very sensitive to the un
derlying magnetic Hamiltonian describing the spin dynamics. For the usual H
eisenberg description of undoped Cu-O planes we find, in accordance with ex
periment, a two-magnon peak followed by high energy sidebands. However the
relative weight of the sidebands is too small to reproduce the experiment.
An extended Heisenberg model including a sizable four-site cyclic exchange
term is shown to be consistent with the experimental data.